Here's how to transfer your Firefox bookmarks:
First, install Firefox on your new PC. (You might be able to copy the set-up program from your old PC, if you've still got it, or you could just download it again).
Then, on your old PC, run a search for 'bookmarks'. You'll find several files but most will have dates on the end of the file name (e.g. 'bookmarks-2008-03-05'). There will only be two simply called 'bookmarks'. One will be tiny (7kb); that's the 'empty' file which comes with Firefox. The other one will be much larger. That's your current bookmarks file.
When you've identified the correct file, you need to transfer it to your new PC. If it's under 1.44Mb you could use a floppy. Or you could email to yourself from your old PC and collect the mail on your new PC. Or you could use Lostboy87's gizmo.
On your new PC, with Firefox installed, again search for 'bookmarks'. Identify the correct current bookmarks file (in the same place as the one you've just copied from your old PC). Right-click on the file and select 'open containing folder'. Delete the bookmarks file, then drag the old one in to replace it.
The big retailers (such as PC World) won't sell you a PC with XP; they'll only offer Vista. Independent retailers might be more flexible (simply because they might still have some copies of XP on their shelves).
Chris