40GB
I have 1GB free on my little 5GB drive. Have you tried tidying it up? Removing all those old programs. Emptying the temp folders.
Start... Run... cmd
del *.tmp /s
del *.dmp /s
Like Stuart says - you just connect the external drive and hey presto. Just remember to use Windows to disconnect it first, otherwise you can lose and/or corrupt data. And you can transfer an internal drive to another PC - just a bit involved than moving an external drive.