I have a 80GB hard drive on a 400MHz K6/II system with a VIA VA-503+ motherboard. Sadly, the 80GB HD is limited to 32GB because of the BIOS. I currently have Slackware 10 installed on all 32GB with reiserfs. I want to add a small Windows XP partition on the disk so I can dual-boot (so I can play games such as Deus Ex). My question is, can I repartition the drive without having to erase the reiserfs partition, thereby requiring me to reinstall Slackware?

And yes, I realise I'd have to reinstall grub or lilo.

You can resize ReiserFS with the help of "resize_reiserfs", and don't forget to read "man resize_reiserfs"!
[for more info read this: http://linux-ntfs.sourceforge.net/info/ntfsresize.html
steps are just the same]
Though if your reiserfs begins from the beginning of the disk, and since Windows requires some space at the beginning of the disk, you won't be able to move your partition a bit farther from the beginning. I.e. "resize_reiserfs" can't move partitions.
BUT!
If you have enough space on ReiserFS partition, you can shrink it as described above.
Then you'll have free space behind this resized partition.
You can create ReiserFS partition in this "newly created" free space
Copy all the info from the first ReiserFS to the second one.
Then you can delete the first ReiserFS and create fat32/ntfs there.

I.e.
Now you have:
disk[ReiserFS 80GB]
After what I described you'll have:
disk[free space, ReiserFS<80GB]

I hope you understand my bad English

PS: Read manual CAREFULLY!!!
It's not easy at all as it may seem for first time!!!