Well there can be many reasons, but one of the most common is "lack of maintainence" on a PC, and controlling what is running (in the background) so that your PC doesn't essentially get overloaded.
Here are some tips that will help you "clean up" your PC and control all those ".exe" files that software manufacturers always want running on your PC. This talks to a "slow PC", but preventing lock up, the BSOD, and general all around slowness is pretty easy and the steps below will help immensely.
A major cause of slow PC's is fragmentation of the hard drive. You need to defragment the HD at least once a week if not more depending on how much you use your computer, and what you do with it.
One way is to use the defragment utility built into Windows XP. It is slow but does the job rather well. Go to:
Start > Programs > Accessories > System Tools and then click on System Defrag to run the tool. It may take several hours, so let it run overnight or simply walk away from your PC for a few hours.
A great program for defragging automatically is Diskeeper Home. It costs $30 but is well worth it. Go to http://www.download.com and search for Diskeeper. Read the Publishers Description and the Editor and User reviews. It can be scheduled to run automatically as the program deems necessary, and if you are using your PC and the program launches itself, it will take a backseat to whatever you are doing and give you the priority for CPU usage. I think you'll agree it's a good program to have. It precludes your needing to remember to defrag at regular intervals.
Another major cause of a slow PC is programs that are running in the background. Software manufacturers write ".exe" files that help launch a program faster when you click on the icon. These .exe files are running all the time unless you shut them down, and they eat up your RAM and can leave you with precious little RAM to run your PC. The best thing to do is to shut down all the unnecessary programs that are running unless you specifically NEED them to be running "in the background". If you can wait another 2 or 3 seconds for a program like Word, or Adobe, or whatever to open then don't allow some of these ".exe" files to run. Each one you shut down, saves you RAM to help run your PC more efficiently. The following program, Autoruns, will allow you to control what runs when you boot up your PC.
Here's a really good Microsoft tool. It’s called Autoruns and it was written by SysInternals who were recently bought up by Microsoft. Before you download the program create a folder under program files called Autoruns. Leave Windows Explore open. Launch Internet Explorer and go to the following Microsoft web page. http://www.microsoft.com/technet/sysinternals/utilities/autoruns.mspx
Go to the bottom of the page and download the Autoruns program. After it downloads unzip the file (You'll need WinZip or WinRAR to do so). You will see 4 files. Drag and drop those 4 files to the Autoruns folder you created under Program Files.
Once you have the 4 files in the C:\Program Files\Autoruns folder you can double click on the file titled “autoruns.exe” (not the autorunsc.exe) to launch the program.
This will bring up a window with several tabs at the top. Click on the “Logon” tab. It will list the programs that are running on your PC and eating up resources. Look them over carefully and turn off any that don’t absolutely have to be running. Simply click on the “check mark” in the little box beside the program name. Clicking on it will remove the check mark. Look for any program that your are not using at this moment that has a check mark in the box. Look for programs such as Microsoft Office, Adobe Acrobat, etc. Once you have edited the Startup list, click on Apply and then on OK.
NOTE: Look in the publisher column and leave items that show Windows; System 32; Program Files; etc. alone. Also leave things that mention Audio, video, etc. alone. Shutting them off may cause some things not to work. Once you have exited the window, reboot your PC for the settings to take effect.
WARNING: Don’t go into the other tabs and start deleting/unchecking things. You will probably give yourself major headaches if you do that. Stay focused on the “Logon” List only. If you happen to shut off something that you didn't intend to, you can relaunch the Autoruns program and simply put the check mark back in the appropriate box, and then reboot the PC. The "program" will be reinstated to run at boot up.
Regularly clean up your browsing history, files, cookies, temp files, etc. A good program to use is Ccleaner and it is FREE. Get it here: http://www.download.com
Also, you need a “registry cleaner”. The registry can get “defragmented and clogged” with various data that you simply don’t need. The only effective way to clean this massive list is to use an automated cleaner. Go to http://www.download.com and you can search for Registry Cleaners. There should be several FREE ones there for you to use. One I've found to do a good job is named Registry Distiller.
Another good thing to have is a little program called FreeRAM XP Pro. It is free and is available from http://www.download.com This program puts a little RAM meter on the System Tray and shows how much RAM you have remaining that you are not using at any given time. I think you may be surprised at how little RAM you have to run other programs. Also, once it is installed you can right click on the little meter, then click on RESTORE and it will open the RAM display window. You can tweak the settings so it will display your CPU usage also. Then just click “Minimize” and then “To System Tray” to return it to the Task Bar.
What happens when you run out of or run real low on RAM is that the OS recognizes the low or no RAM situation and starts using a section of your hard drive as memory. This is called virtual memory, or a swap file. The PC is constantly writing and reading from your HD. Comparitively, it is a very slow process. Check your motherboard manual and determine how much RAM each slot can hold. It will also tell you what type of RAM you need. You can also go to http://www.crucial.com or http://www.4allmemory.com and run their analysis utility to determine what kind and how much RAM is installed in your PC. Depend on your MoBo manual to determine the RAM limits of each slot though.
Hope this helps!!!