It could be a virus, download and run these programs,
1. Malwarebytes. Malwarebytes.org
2. CCleaner. (backup registry when prompted
). CCleaner - Home
Follow these tutorials. System Files - SFC Command Repair Install For Vista
This will ensure that there is no programs on your PC that should not be there, and will also ensure that your file system is clean and not corrupt in anyway.
If the problem is still there after this then at least we know the system is clean and can go from there.
As JMH mentioned, the more information you can give us the better.
Hope this helps, give it a go and post your results, good luck.