you will need to boot into Safe Mode. To do this restart your computer and repeatedly tap the F8 key as it begins to reboot. You will eventually see a screen that offers several boot options, one of those is "Safe Mode". Select Safe Mode using the arrow keys, then hit enter. It may take a bit, but eventually you will see something greatly resembling the Windows opening screen. As your display will look odd to you, please don't concern yourself with that. There is a reason for it and it should look odd. If you are requested to enter a User Name, type Administrator. Hit enter. Once XP is fully loaded (and it will still look odd), open Control Panel. Then open User Accounts. Then select your normal account and view the properties of it. If it has been set to User, change it to Administrator (or Power User if you have that option). Close everything and reboot into normal mode.
If you are running as an Administrator, and have been in past, I hope you are using a good firewall and sitting behind a NAT enabled router.
If you would like further help with this, or have questions as to my previous paragraphs, I will be happy to help you. Please privately message me in this case. You can do this by clicking "Send Message" at the bottom left of this post.
It does sound as though you may have a virus, however. While you are in Safe Mode, and if your virus definitions are up to date, run a full system virus scan and save a log of the report. See the link below.
http://www.symantec.com/security_response/writeup.jsp?docid=2003-090717-4224-99&tabid=1
My apologies - I missed the fact that shut down is missing from your Start Menu. You will need to do a hard shutdown (using the power button) and then use the power button to turn it back on. As it comes up, tap the F8 key and follow the same instructions posted earlier in this reply.
edited to add last 2 paragraphs
