Sorry to state the obvious, but I very much doubt you'll get much enjoyment out of GTR 2 (being the demo or not) when using keyboard instead of a wheel setup.
I started off trying to use a combination of joystick (for steering) and keys (for accell/brake & gears) and it was kind of workable in Semi-Pro difficulty but you really need pedals to get the most out of the game and play it on Simulation with traction control off.
If you're looking for a cheap(ish) wheel then I'd recommend ebuyer.co.uk:
http://www.ebuyer.com/customer/search/index.html?rb=20884577697&intSubcatUID=1077&strSearch=steering+wheelBut if you're willing to splash out then the most popular wheel (and arguably best) seems to be the Logitech Driving Force Pro. If you're willing to splash out for one of them then see
this thread. It's not officially supported on the PC but there are drivers available which I'm assured make it work perfectly with windows XP (mine is to be delivered tomorrow). See that thread for full info and driver link.
Hope that helps and if not hope you're able to enjoy the game with keys if someone with more intelligence knows of a way