I burned some PowerPoint, Word and Excell files with Nero Express on my PC with Win XP Pro. Then i can't read the damn' (excuse me) CDs on my Mac with OS9.2 installed. I can't either read them onto my PC with Win 2000 Pro, but that's another problem. I used to Burn Hybrid (PC & Mac) CDs with my Mac, but i can't seem to find the options in Nero Express for that and the Online help is no good either... Any idea would be grearly appreciated, i'm in a big big HURRY !!! Thanks in advance !

It might be the Mac hardware - did you finish the session/close the disk? I know that my Dad's laptop can't read unfinished disks at all. Also, make sure all the ISO settings are on the highest compatibility modes (even if this means that filenames are forced 8.3 and uppercase - at least they'd work)!
In Nero you can mount a Mac volume to burn as a hybrid, but as you say "Nero Express" and "Can't find it" I don't know if your version supports it...

I would say Ben has some decent points, especially after reading this even though it is the other way around.(Mac->PC burning hints towards the bottom), it does give some good hints as to what is going on. Hybrid disks on Mac are actually the default, I think. Although I burned straight iso files, and it worked nicely in Mac OS X, and have seen isos burned on windows machines readible by Mac OS 8/9 as well, so as long as you use standard images, and close the session, you should be fine.