This and other types of weird things happen when resources are too low or when a system loads a lot of programs at startup. Do you have a lot of programs starting with Windows? Have you did and MSCONFIG yet?
Here's how if you're not sure... It's safe.
First thing I'd do is reboot the system, right click My Computer, click properties, click the performance tab and check your resources.
**If it boots with less then 80% free then your system is starting with a good chance of acting weird before you shut it off again. With just a virus scanner (which is all I have running at startup at the most) you should be in the nineties or upper eighties.
close the properties dialog box. Click start, click run, type MSCONFIG, click ok, click the startup tab, uncheck everything, click ok, click yes to reboot, see if you still have the problem.
Your system will boot fine with nothing checked. You can go back and check any box next to things you want to start when windows starts after you've tested.