wintrust.dll is a process that uses functions to verify trust in files, catalogs, memory blobs, signatures, or certificates by third parties. A checksum is a string of characters that checks to see if the file is corrupted or not.
Since the file is related to security, it's likely that a virus tried to replace or corrupt the file on purpose. Since you can't log in to do a virus scan, we need to get creative.
There are ways to access your important documents if we cannot get your computer up and running again. But first I will list ways to fix your computer and get it running.
The most direct (and easiest) way to repair the file is to boot your original installation disks and run the automated system recovery tool. To do this, boot from it, wait until the drivers load (do not go into the recovery console, this is something different) and continue as you normally would to install Windows, except after it detects versions of windows, press "R" for repair, and it will initiate an automated recovery.
Most people do not keep their original install disks, so another way to do this is to use a Linux boot disk to mount the Windows partition and replace the wintrust.dll file. My guess is that you aren't familiar with Linux so I will try to be as detailed as possible.
1. Download a disk image. Here is one (which is free), it's called Ubuntu. There are tons more you can use but this one tends to be most popular and the easiest to use. http://www.ubuntu.com/getubuntu/download
2. Burn the ISO image you got to a disk. Make sure to choose the "burn ISO image" option, not a data disk or anything else, or it won't work.
3. Boot into the disk just like you would a normal windows install disk.
4. When it's done loading, from the menu choose "boot from disk without changes".
5. When you are on the desktop, go into the menu at the top and look for a program called "terminal". it should be somewhere in the accessories menu.
6. Now you'll see a white box with black text and it should read "ubuntu@ubuntu:~$"
7. Type the following EXACTLY and press enter after each line. If it says "command not found" or "syntax error", try again (don't type the ubuntu@ubuntu:~$). You may browse to this question with the firefox browser in Ubuntu and copy and paste the commands if you wish.
ubuntu@ubuntu:~$ sudo bash
ubuntu@ubuntu:~$ mkdir /mnt/windows
ubuntu@ubuntu:~$ mount -t auto /dev/hda1 /mnt/windows
[NOTE: If you get an error after this command, then hda1 isn't your root drive. Try "sda1" or "hda2,3,4" or sda2,3,4 etc until you stop getting an error]
ubuntu@ubuntu:~$ ln -s /mnt/windows ~/Desktop
8. Now a link to your Windows files should come up on your desktop and you can stop using the terminal. Download a replacement wintrust.dll (I recommend one by Microsoft, from somewhere else might be malicious) and brows to it's location. Replace the file. Reboot back into Windows
The second step is the most difficult and it is understandable if you need to try it a few times. The terminal is not something for casual users.
If, for some reason, this doesn't work, you might need to just get your files off the computer. To do this:
9. Follow step 7 again.
10. Hook up a USB flash drive or external hard drive to the computer. It should recognize it automatically.
11. Open both locations and find the files in your Windows folder you wish to keep. Drag them to the backup device. After that, go ahead and reinstall Windows.
[NOTE: if it says "permission denied" or something similar, do the following:
1. Press F1. The screen will blackout into a terminal.
2. Type the following exactly:
ubuntu@ubuntu:~$ sudo bash
ubuntu@ubuntu:~$ killall gdm
ubuntu@ubuntu:~$ startx
And the desktop should reappear. All this command did was log you into the administrator acount so that you would have permission to access the files.]
I hope this helped, good luck.