Only Able to Logon On as Guest Account

[Guest]

This may be a coincidence but I installed a new anti-virus program on my
laptop and and rebooted. When it restarted, I was presented with a user
login screen and the only available account was "Guest". I have never
previously had more than one user account and it always booted straight into
XP using my default administrators account called "Peter".
I don't know how to recover the situation as when I login as Guest now I
have no admin rights. I can see the "Peter" folder still exists under
Documents & Settings and I really don't want to re-install XP Home if at all
possible.

 

[Fitz]

Boot to Safe Mode and the default "Administrator" logon on will appear.
After logged in, change your account type to administrator.