Windows XP refuses to start

I'm using a Dell Dimension E310. I recently upgraded to SP3 on my windows xp. Now It can't start up, and gives me this message:

Windows could not start because of an error in the system. Please report this problem as:
Load needed DLLs for kernel"

I've looked up a few sites on how to fix this error. Everyone keeps suggessting reinstalling windows. I don't have a windows installation CD. I got this computer 7 years ago.

Now the difference with my situation is that I have both vista and XP on my computer. So I'm still able to access the files of my XP through vista. Now I hate vista, and I would like to get my XP working again because Most of my work and programs I use are on that side.

My question is this: Is it possible for me to fix this problem without reinstalling if I am still able to access the files ? If so, Please let me know.

Hopefully I just need to download a certain file and insert it in the right folder

Thanks.

"I got this computer 7 years ago"... 7 years is very old for a Dell, the motherboard, hard drive and/or power supply can be toast. There is no way to replace just one file, that's not your problem anyway. You can use this computer as a centerpiece or paperweight