This is a very common topic and problem many people have. So I'll put down a way to get it fixed. If you like you can also show your way of fixing the problem as their are several ways to. I hope this works for you.
1. Install NFS Patch 4.50 from http://www.nd4spdworld.com - while installing, the program will ask you where you want to install the patch, you must click on the 'Browse' button and search and select your NFS HS directory. After that, it will automatically download the patch to the seleceted directory.
2. Install NFS Faster Graphics Patch 1.0 from www.nfscars.net - during installation, you will come to a window which will show 'Nfs5 (new)' and 'Nfs4 (old)', DESELECT the 'nfs4 (old)' one and leave the other one. The program will again ask where to install the patch. Again, use the 'Browse' to search your NFS HS directory.
3. Install the cdpatcher_v2 from nfs2000.de - during the installation, like the two above, it will ask to search for the NFS HS directory. NOTE: This site is in German so I'll give you the directions to the patch:
Type www.nfs2000.de on your Address Bar > On the left side of the site, you'll see the nfs games > Scroll down to until you see NFS HS on the left > Below the name, there will be Cars, Tracks and all that, select Tools > In the middle of the site, you'll see (at the top right after the top banner) 'Trainer/Patches Updates' or something like that, click it > Scroll down until you see the patch and click the link > Click the download link on the pop-up.
NFS HS should now work on your XP :) .
But before you install any Hi-poly cars, Install the NFS Hipoly patch from any NFS site.
It only works for career, but if you're lucky then it might work for arcade mode too.
Thanks! I've been meaning to post this :)