Can I use both internal HD and external HD as main?

I installed Windows 7 on my ASUS EEE PC 701. It runs great, but the only problem is space on the hard drive. The internal hard drive only has 4GB, and about 3.20GB of it was used to install Windows 7. I'd like to install programs like iTunes and Adobe Audition but one of those alone would probably exceed the amount of space I have left.

However, I do have an Western Digital 320GB external harddrive. How do I make it so I can switch between using the internal hard drive and the external one as the main storage for installing programs and such? I know its possible because I've seen many people ask about it, but nobody ever completely shows how to do it. They just vaguely explain how it works...I'd really appreciate detailed instructions.

Thanks in advance!

I'm not sure how you'd go about it without problems. You would always have to have the external HD attached for Windows to work, at which point you're better off getting something suitable for that use from the beginning, i.e. a desktop PC, or a regular notebook with a proper HD. You can't just arbitrarily be switching from Programs residing in the Eee PC's internal drive and an external drive.

My 2 cents :).