Problem re-installing XP

but please help.
After a week of increasing glitches and hangs my computer internet LAN link
failed then the computer locked up in the Zone Alarm start phase of set up.
I could not get it to start in 'safe' mode so I decided to try to start it
off the XP CD ROM..Ok so I probably made the problem worse by doing this.

That worked and the computer is up but I cannot open my old files - access
denied. This is a BIG problem as my security & WLAN encryption codes are in
those files so no internet, no XP registration, no firewall. Also I cannot
start some programs e.g. Office. I would be happy to completely re-install
if I could get to my files to copy back up for the last months work.

Now I may have used a different 'computer name' this time. SO how can I
open those files saved under a different computer name? or can I re-name the
computer now?

Grateful for any advice
.