As "fender" said try ebay for an HDD.
For an OS check this out (totally FREE!)
http://www.linuxmint.com
IMHO superior in everyway than anything that M$ supply (at a cost)
You will not believe how fast your old pc can go.
As a case in point I use a version of Linux on a laptop of 2000 vintage and it is way faster that my newish desktop running XP