Check your user profiles stored on your PC.
Do this by right-clicking My Computer, select Properties, then click on the Advanced Tab. Click Settings under User Profiles. Check that there is not an unknown account, and if there is, delete it.
Also, XP likes you to log in as an Administrator, not just a user with Administrative rights. The Administrator account should be built in (if you have done a clean install), but if you have a PC that came with XP on it, go into User Accounts in Control Panel, and create a new user account called 'Administrator' (with administrative rights), and use this account when installing your software.