Any idea why this happened and what may have caused it?

Just a guess but I'd say it was caused by spending all day "installing and uninstalling different glide wrappers". Some of them may have gotten left behind, in the registry for example, and when XP tries to initialize them it freezes. They could be conflicting with each other also. Make sure you have uninstalled all of them completely. If you haven't yet try running System Restore to undo any of the changes you made.

If this happens again is there a way to get past the frozen screen?

Press F8 right after the POST screen to get to the boot menu and start your computer in safe mode. If even this doesn't work you can use the CD to boot into the recovery console.

I have an xp boot disc - which i believe boots in to pc- dos. using this i get a black sreen with the a: prompt.

This is not DOS, it just looks like it. It's a 32-bit command console, and you cannot install XP from here. To reinstall XP boot from your CD and either choose repair or go ahead and delete your existing partition so you can do a fresh install. Back up anything you want to keep first of course.

Thanks for that Banquo - the same thing has happened again - i was unable to boot from safe mode but could boot from debugging mode - here my soundcard drivers and directx version were both older ones -I have used glidos and not installed or uninstalled anything since the last time this happened.
Please if someone could give me an idea on what may have caused this and how to stop it i would be very grateful. Also what is the difference between safe mode and debugging mode?
Thanks .

Something similar happened to me; my video drivers had somehow become corrupt. I solved the problem by booting with a borrowed video card and reinstalling the drivers. No guarantee, just something to try.