Problems switching between accounts

This started yesterday and I'm not even sure how to google for this one.

Windows XP stays on my account most of the time (Main account). We also have an account on here for my father.

The last two times he accessed his account- it was fine in switching - did his email and whatever else. Upon switching BACK to MY account though- a BLACK screen pops up...and you can't DO anything- or anything that I can find.

Most times I have my OE open and some of my IM Programs logged in but set to "invisible/appear as offline" when dad usually checks his mail.

What could be causing the black screen when he switches the accounts back?
Are the IM programs being logged in interfering?

I did install a new CD/DVD burner and a new Logitech mouse recently but no software was installed for either one.