If you can log in with admin authority, I'd create a new user ID (via Users applet of the Control Panel), copy whatever I need from the other IDs and delete them. Then I'd boot to Safe Mode and check for viruses.
Good luck.
I have inherited a computer which has two users installed but neither have their own directory structure created by XP for some reason... all have xyz and this is what the machine prompts for at login - even though it does not appear to have a user called xyz.
Have I been hacked and how do I change this situation?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic