Go to Start/Admin tols/Computer management/Disk management
Thats only any good to you if you had created a partition smaller than the full drive size when installing XP as you would have had some unallocated space which you would have to use the disk manager to create a partition and allocate it
If you want to partition your 80 gig drive which already has your XP os on it then you will need to run a program such as partition Magic to resize or create the partitions how you want them.
I cant think why your pc should be running slower with the 80 gig drive in than the 20 gig one unless you have something set to check the drive as its booting up
If you havent got too much on your new drive that you need to keep then I would just reinstall XP on your 20 gig drive as a master and set your 80 gig drive as a slave to it.
Dont forget to set the jumpers of each drive correctly for master and slave