Computer Log-In Problem?

I have a windows xp computer and i just put a password on it that I still remember. I tested it after I set it and I logged in successfully and I came back a couple of hours ago to log in but it didn't work. I still remember the password and I have tried multiple times to log in and have also restarted the computer multiple times but have been unsuccessful at logging in. Any ideas on how to fix it. I don't think i have a virus because the computer runs fine and I scan it often.