how to expand virtual drive?

i have parallels 5 and run windows xp. my virtual drive has less than 5 GB left of 40 GB and things are running slower and slower. my mac (snow leopard 10.6.4) has 750 gig free.

my question is how do i expand the windows drive, and in doing so, keep everything the way it is in windows (i do not want to have to reinstall etc).

i have tried looking through the various forums, but have not found an answer.