Tell more:
- what's your OS?
- when does the problem occur?
- if it occurs at start-up: can you boot in Safe Mode?
- what antivirus are you using?
- already tried system restore (Windows XP) or scanreg /restore (Windows 98)?
- did anything noteworthy happen just before this started?
- do you have an OS cd and drivers (or a recover cd) in case you have to reinstall the OS?