Slow computer syndrome could be caused by a whole bunch of different reasons. Here is a list that you should consider checking through. This contains both the problem and suggested resolutions. The site Speed Up Your Slow Computer gives you more detail on this.
Problem : Hard Drive Is Too Full And You Need To Free Up Disk Space
Solution 1 : Purchase a new hard drive and transfer your files
Solution 2 : Uninstall programs and applications you don't need
Solution 3 : Search for and delete files that you don’t need
Solution 4 : Search for and delete duplicate files
Solution 5 : Backup outlook files and save to another storage device
Solution 6 : Run the disk cleanup utility tool
Solution 7 : Move large files that you seldom access to another hard drive
Solution 8 : Clean up Service Pack and Windows Fix/Patch installations
Solution 9 : Delete temporary files
Solution 10 : Delete temporary internet files
Solution 11 : Remove system restore points
Solution 12 : Compress files
Solution 13 : Delete the memory.dmp file in C:WINDOWS or C:WINNT
Solution 14 : Run the "compact pst" feature in MS Outlook
Problem : Hard drive is fragmented
Solution 1 : Purchase a defragmenter to defragment your hard drive
Solution 2 : Use the windows defragmenter to defragment your hard drive
Problem : Registry is corrupt or has errors
Solution 1 : Purchase a registry cleaner to identify and resolve errors
Problem : Memory is overloaded
Solution 1 : Stop background programs from running
Solution 2 : Upgrade your computer memory
Problem : Computer is infected with adware/malware
Solution : Use a scanner to find and remove adware/malware
Problem : Computer is infected with viruses
Solution 1 : Scan for and remove viruses
Solution 2 : Block viruses from coming in
Problem : Your operating system settings are suboptimal
Solution 1 : Change memory.dmp settings
Solution 2 : Switch off hibernate function
Problem : Your hardware drivers are out of date
Solution : Update your hardware drivers
Best of luck.
Ben