Security Certificate Alert on Boot Up

I have a Windows XP SP2 and everytime I restart right after the desktop icons load a pop up displays about security certificates, which I have attached a picture of. If I click yes or no it goes away and the computer seems to work normally but its really annoying. If anyone can help I really appreciate it. Thanks guys.

1. When did this start? If suddenly with no previous software changes, do a thorough anti-virus/spyware check at once. 1st item on the 'security and the web' forum here.

2. Oh, might sound stupid, but is the computer date and time correct?

3. Maybe you have installed some software which uses security certificates, and which you have neglected to stop 'phoning home' for an update every time the PC boots. If you do not have an internet connection at boot time, the certificate cannot be validated. Run msconfig and see what is in the startup and services tabs.

4. On the other hand, I seem to vaguely remember this can be something stupid like an overloaded temp directory, or internet explorer cache. Run drive cleanup from the general tab of your hard drive properties panel. Clear the explorer cache.

5. The worrying thing is why at boot time ? Usually this sort of thing only occurs when you access certain websites with your web browser, or maybe your ISP has failed to update their certificate. What internet connections do you open along with Windows startup ? Do you have email/chat/AOL/ etc etc starting with Windows?