it'll be rather long winded and you'll need the following:
-preferably a blank CD
-some patience or a fast internet connection
I'm going to explain how to download Ubuntu Linux, which is an operating system that can be run as a live cd (no installation)
1) go here:
Download | Ubuntu
2) click the big orange download button (no need to change any of the drop down info)
3) download and save the .iso file (it's 693.2MB)
4) once download has finished, burn the iso to a blank CD (if the other machine is also Windows 7 (may also be in vista), you can use the built in iso burning tool. if it isn't, then I'll tell you an alternative program)
5) once disk has finished burning, switch computer off
6) put disc into the problem computer and switch on
7) press whichever button takes you to the BIOS boot options (before error shows, the bios thing will be the first thing you see)
8) select the CD drive
9) three lines of black writing will scroll across the screen saying stuff like "isolinuz" etc. this is normal.
10) it will probably go to the 'would you like to install or run the livecd' screen. select live cd.
11) wait for the desktop to load.
I'll post the next steps if you choose to use this method (it'll take me a while to remember what steps need taking for the hard drive diagnostics)