|As I said, most likely some program is conflicting with it. You could try tempoararily disabling things in msconfig Startup tab to see if you can unearth which one is causing it.|
There is a vague possibility of a virus, so you could run this freebie on it, which often finds what normal AV's miss (green button top right):
If it finds anything copy/paste the log on here so that we can see what's going on.
Always pop back and let us know the outcome - thanks