Luckily I managed to somehow fix the problem, here's what I know so that if anyone runs into this perhaps this can help them out.
I tried booting in safe-mode first, that did not work though. Next a friend suggested that I try plugging in the external keyboard
after I see the login screen. This actually seems to have fixed the problem as I was able to bring up the alternate login screen using Ctrl-Alt-Delete, and once I logged in the trackpad started working again.
Some interesting things happened after that. Windows gave me a pop-up saying it had experienced some sort of severe error:
BCCode : 100000ce BCP1 : BAB8178C BCP2 : 00000008 BCP3 : BAB8178C
BCP4 : 00000000 OSVer : 5_1_2600 SP : 2_0 Product : 256_1
C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\WER919e.dir00\M ini041507-01.dmp
C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\WER919e.dir00\s ysdata.xml
I have no idea what that means though...
The 1.2 Beta installation had installed an Apple Software Update application which I ran, and upon attempting to install Quicktime it told me that the 1.2 Beta bootcamp installation had been suspended and said that if I wished to proceed I would have to click "Yes" to undo that installation. I did this and Quicktime installed just fine.
Now everything seems to be working normally just like it had before. I'm not sure if any damage has been done or if I now have a partial 1.2 beta installation, all I know is that I'm not touching those updates while everything is working. I will wait for the final release instead. :-)
I'm running Windows XP Pro BTW, not Vista.