Try using System Restore to get back to just before this problem happened>
You will have to try to start up in
Safe Mode if you have tried, well, you didnt tell us that! If you need directions, they are below, the same as for getting to the "Last Known Good Configuration" entry.....
If there was a major problem at that point, such as a virus or other going on, and you would rather not go back to it, let us know.
You would usually have a Restore Point just a day or so ago and depending on just when you did the file replacing, a Point just before that would probably be OK to use.
Here is how to do that:
http://bertk.mvps.org/
Try the "Last Known Good Configuration" bootup available when you use the F8 key at startup, before you see the XP logo, start tapping F8 just as you start, at the menu, with your down or up arrow key, move to "Last Known Good etc..." and press Enter once to try that.
If you have access to another pc running XP you might be able to put your hard drive in that as a Slave drive and copy the right file out of your .dll cache, or from the Desktop back, but I am not confident that would work. And, you need some knowledge of working inside the PC and how to hook up a Slave hard disk, jumpers, cables, how are you with those things?
NOTE>>>>You DON'T want to try to boot from your drive while it is in another XP machine!!!!!
There are other things to try, such as booting from your XP installation CDROM.
You can access the Recovery Console with that and move or copy the file back to it's correct location, or run the system file checking tool (chkdsk /r) are you at all familiar with that?