having a drive partitioned can be handy.
for example, install all OS stuff (windows) and software on one partition, and have the other for your data.. all your personal files, music, etc.
if anything goes wrong, just erase the partition with the programs on it and re-install everything. but you still have all your data. its a lot easier than having to try and backup all your data first, if your computer is in a state to let you at that point (say if it got some terrible virus or some other catastrophic failure)..