Make sure that your password is correct, number lock is on if you use numbers in your password. Capital letters doesnt effect the username, but it does effect the password. Make sure that the password if case senitive, is eneter correctly.
If that doesnt work,
Press F8 after the computers starts (Just before windows starts)
try using "Last good known"
OR restore system function in safe mode.
Micrsoft has started checking for pirate copies in part of the world. When you do an update, some systems will not start(SP2), some will get a message stating that you have 30 days to purchase a license. (All this is new and will effect about 50 countries)