Hi, I am perfectly able to join any project I want except gpugrid. I really don't see what's the problem. I have been searching for a solution for quite a time, but with no results. When I type my email address and password(which is correct - as I said I can join any other project) it says the same as if I would type a wrong password in some other project join dialog, that is I should check my email address and password. Can anyone tell me, what might be the "catch"?

Perhaps you have already joined GPUGrid and used a different password or email address from the one you think you used?

Do you have an NVidia GPU installed on your system?

Boinc may also have caused an error that would require you to uninstall it, restart, clear the registry, and then install an updated version.

1) I have never managed to connect to GPUGrid.
2) I do have nVidia GPU on both my computers(8800GTS, 260GTX)
3) I have tried to run GPUGrid with my account on many computers, not just the two. I use dual boot on both my computers and I was not able to connect any of those OS's to GPUGrid(XP, Vista, Win7 64/32bit). In addition, I reinstalled my system Yesterday, and tried again... in vain. It must be something with my account. But what exactly?

You could always create a new account, or change your account details on your other projects and then use those new details here.

Indeed, but the idea of simplicity lies in having just one account, doesn't it? Guess I don't have much of a choice though...

You will get recognised here for what you do here, and will get recognised for what you do elsewhere in various other forums and threads.
My accounts are rather messed up too – My WCG account and several other long gone projects seem to be disparate from the global catalogue. So I am missing out on some 1.7M points on some meaningless global stats forums.

Anyway, this is the place that really counts.

I don't get the last sentence : "Anyway, this is the place that really counts."
I'm not native speaker as You might have noticed, but the thing is I think my GPU's have much larger potential then both my CPU's together so I AM going to make a new account. Hope it helps...
Thanks.

That was my point. This place is important because if you have a good GPU it will do a huge amount of work compaired to a CPU.
Good Luck,