After the POST screen appears push F* and hold it down until you get a black screen with white writing on it with different ways to start Windows. Chose [b]Safe Mode[/b] and see if it starts up properly.
If it does you can then run your Anti Virus Software and some Mal ware scans over the computer to remove any infections and try rebooting normally.
If that doesn't cure your problem you can put your Install CD or Recovery CD into the CD/DVD Drive and boot from it. You then have the option to reinstall Windows or Repair Windows. You can try the Repair option but honestly I haven't had any success with it so I would recommend reinstalling Windows without Formatting the HDD that way you can recover your Data but you'll have to reinstall all the software that runs on top of Windows.
Once you have your Data backed up you can then wipe the drive with a wiping utility and do a clean install of Windows. That way you know you have an uninfected OS to work with.
It would also be a good time to instigate some kind of Disaster Recovery Plan as computers [b]Are Not[/b] unbreakable and you always should have a recent Backup available in case you have some hardware failures, particularly HDD dieing as to recover the data off a dead HDD can cost you thousands of $.
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