Many, if not most, Windows computers are used by a single person with a single userid. Even shared machines are likely to have a single Windows logon. Defensively speaking, this is not good. Anything can break, including a Windows userid.
Twice I've worked on Windows machines where just after logging in with a userid and password, I got logged off. Like the Soup Nazi from Seinfeld, Windows was saying, "no computing for you." Because I always create two userids on each machine that I maintain, this was no big deal. Windows itself wasn't brutally broken, the problem was limited to user1. Logging on as user2 worked fine.
To give yourself a fall back position, create a new Windows user. Many people logon as an unrestricted administrator class user, so the following instructions create a new unrestricted user in Windows XP:
Control panel -> User Accounts -> Create a new account
A good user name would be "backupadmin". Click the Next button and make sure the account is a "Computer administrator", then click the Create Account button. This leaves you back at the User Accounts home page. Now, click on Backupadmin in the list of users and opt to create password.
You now have a spare tire in the trunk, so to speak. If, in the future, you can't log in to Windows, or something else strange happens, you can try to log on as user Backupadmin. It might fix the problem, it might not, but it can't hurt. And , as a side benefit, if each user has a different password, yoiu get a second chance should you forget your password.
Windows does some first-time processing when a user logs on initially, so I suggest logging on as user Backupadmin once just to prime the pump as it were.
If your copy of Windows XP normally boots directly to the Windows desktop, without asking for a userid or making you chose one by clicking on it, you may find that with the new userid, Windows now boots to the logon screen, where it displays a list of the available users. If you'd rather boot directly to the Windows desktop, then click the Start button, click on Run and enter the below in the white rectangle that is confusingly labeled "Open". Then click OK.
This opens a window called User Accounts. Click on your desired default userid to highlight it with a blue background. Then turn off the checkbox at the top that says "Users must enter a username and password to use this computer". To make the change effective (either the Apply or OK button) requires entering the password of the default user.