The article above explains how to use System Restore from "Safe Mode with a Command Prompt" The problem is, you still need to login to an Administrator account.
If that doesn't work, then a Repair Install will not wipe the disk and should preserve all your documents and other personal files, but that is not guaranteed. You may need to re-install any software. How to perform a repair install.
Sadly it seems you have not made backups of those fies you don't want to lose. There are ways to extract those files. Some options there;
1] If you have an external hard drive, use a Linux Live CD, (Knoppix Live CD may be easiest), to boot up. This can explore your hard drive and allow you to copy those files across to the external drive.
2] If you have a bootable CD ROM and a 2nd CD/DVD R/RW drive, you can do the same as above and burn the files to CD/DVD.
3] Disconnect the C drive and re-connect it as a slave in a working Windows system, and extract the files.
4] As above, but enclose that C drive into an empty USB enclosure, connect to a working machine and extract the files.
5] Add another drive to your system, install XP, and connect the unaccessible drive as a slave or a USB.