I think that this could be due to an old/new driver clash. Open up Device Manager, click on display adaptors and uninstall the one named after your old video card. Then download Driver Cleaner (google it) and clean up your nVidia drivers, then restart and install the brand new drivers.

This problem wouldn't have come about if you waited and built a new PC from scratch, like I told you to! 

Rob. 

I got into the habit these days that if I do a major component change I re-format.  I use all the latest drivers etc or recommended ones, that way I know I have a fresh clean system. Just my way of doing things though.

Detailed guide to how to reformat your hard disk

This should sort everything out, Nick.

Rob.

I think it does, unless you buy a Win XP disc from PC World or wha'ever  But then that means another £110 out of the wallet...

Why didn't you just buy that new PC? 

Cheers,

Rob. 

Dont panic yet.  I take it its a package PC that you have upgraded.  Lots of these do not come with there own OS disk, but they come with a 'system restore' disk.

You would have to use this in your case.

Restore disks are a personal hate of mine though, they always put a load of crap on your PC that you do not need.  Thats why I build my own systems and have my own OS disk and do it my way!

oh dear....just found this forum, read it and weep....
http://forums.nvidia.com/index.php?showforum=25&prune_day=90&sort_by=Z-A&sort_ke...

the amount of people having problems with 6600 gt's is incredible. only wish i'd read this first.  all the reviews i read on this card couldnt praise it enough, but i can  assure you otherwise. my only problem is what to do now.  i'll try a few more things but if i still have no joy then it will be going back to overclockers....

Dont read to much into forums when there are problems.  The same thing happened to me.  I had pre-ordered my 6800Ultra and before I received it the forums were fillin up with people with problems.  this was only due to it being a new card and a few teething problems.

When I received mine I put it in, loaded drivers etc and it ran perfectly.  Though I did choose a manufacturer that I knew wasn't having many problems.

People use the forums a lot more for problem solving than they do for saying how good their card is.  This is because most people are too busy playing games on their new cards!!

There are a few 6600GT probs there but there seem to be more 6800GT probs which is worrying as that card has been out a long time and most of the probs sorted.

Anyhow Hopefully you read through the 6600GT probs.  Drivers recommended were either 66.81 or 71.24 drivers, both available from www.Guru3d.com

I will tell you the few glitches I knew about b4 getting my card (Unsure if it will help yours though) :

Turn 'Fastwrites' off in BIOS.  It caused major stuttering on my card.  Known problem on the 6800's not sure about yours though

Get a registry tweek called Coolbits.  When you get this and run it, it will add overclocking functionality to your display properties.  Ensure your 2d and 3d clock speeds are the same (again if they weren't performance was lost)  the newer BIOS versions on the later 6800 sorted this, just check yours to be sure.

Any other problem you have will be down to drivers, software setup and that your PSU is good enough.  Have you run DXdiag and checked that all accelerators, i.e. 2d and 3d are working correctly?