Reboot into "safemode" by pressing the F8 key before getting to the Windows screen. You'll be required to supply your administrative password at the log in screen. Once logged in while in safemode you should be able to access the "User Accounts" applet in the control panel.
Once there you should be able to see how many users are registered on the machine and make changes to them (or create or delete them for that matter). XP requires at least 2 accounts with administrative access, the Administrative account, which under most circumstances can only be reached in "safemode"; and the main account which is listed as administrator, but is really a "Superuser" account (using a bit of Unix terminology here) that has most but not quite all of the privileges of the administrator.
Check what kind of privileges have been enabled/disabled for your account. Something may have been corrupted changing those settings. Reset your main account to administrator reboot and check on the results.
If you're still having problems, reboot into safemode and this time create a new administrator account with a different name, reboot and log in to the newly created account. Check and see if this account is functioning properly.
If it's not then you have a more serious problem than I can help you with here (you'll probably have to reinstall Windows), but if it works OK then all you need to do is copy your files and data from your original account's desktop and documents folders and you should be good to go. If after copying everything you want from your old profile, you decide you'd like to save space, you can easily delete it from the User Accounts applet.
I hope this helps,