My 20GIG hard drive is nearly full

[kineas]

I guess you will be able to use your old and new hard drive together.

Yes, you will need to format it.

If you plan to remove the original be prepared for an awful lot of work...installing winxp again, backing up and restoring data etc.

Try to buy a book on this or just google around, e.g. http://www.basichardware.com/HowTo_Insta...Drive.html