ps2 unlike usb is not plug and play. U HAVE to have a ps2 mouse plugged in before booting and if the system doesn't see it, then there may be some setting in your bios to enable it (which is doubtful but possible). Ps2 keyboards aren't as anal as the bios will initialize a keyboard even if its not there which is why u can normally unplug and plug in a ps2 keyboard at will while a system is running, a ps2 mouse on the other hand will lock up the bios if plugged in while running. There may be a firmware upgrade available from Alienware to resolve this also, or you may have a faulty unit, or even just a screwy windows install. USB mouse should just work, no fighting at all unless your using an old version of windows like 98se or something, but I'd doubt that ud be that silly.
You can try booting off a cd that runs a program with mouse support to rule out windows being the culprit. Something like a suse live eval cd would do the trick, or even just some of the hard drive test utilities such as western digital's wddiag (I think they only offer a floppy version but if you know what ur doing u can make a bootable cd out of the image).
Good luck, and remember that percussive maintance sometimes helps :-D
Some people are anal about these things 
