External hard drive found as unknown device

I have a LaCie external hard drive which works fine on other pc's running Windows 7 Ultimate, but on this one it is an "unknown device". I have tried it in different ports but with the same result. I've checked with Windows Compatability and the exact model is listed there as being compatible. I've been to the manufacturer's website but there are no drivers available.
I have the same problem with any USB device I plug into any port. I have tried a complete Windows re-install from a different CD but it made no difference.
The only USB devices that work are the keyboard and mouse.
I have given the LaCie HD as an example because I am hoping that if I can get a specific device to work, I may be able to solve the problem with others.
I've done the upper and lower filter removal thing in Regedit, and have uninstalled and reinstalled the USB drivers numerous times but nothing works. I've also updated the BIOS.
Any ideas please?

NOT a pro ! but i think after what u have done the only thing faulty would be your ports <<<<

I had the same problem with my pc and after a hell lot of time waste i came to know that my ports are out ......

Am not sure whats the case with yours ........

Thank you for your reply. The problem is that it is hard to imagine that all the ports could become defective at the same time. There are 4 at the rear and 3 at the front and they are all the same. Also there is the fact that the mouse and keyboard are still working, and that devices are seen it is just that windows cannot identify them.

U r right ! these are rare cases !
There is a virus also that hides the usb although it is connected . . . .!
YOu might have to re-install windows .... ( IF u can)

beret21 said:

U r right ! these are rare cases !
There is a virus also that hides the usb although it is connected . . . .!
YOu might have to re-install windows .... ( IF u can)

Thanks for the suggestion, but as I explained in my original post, I have done a complete re-install and the problem remains. I have seen similar questions to mine on a number of forums so I know mine is not a unique problem. The things I have already tried are things suggested on those forums, and while they seem to work for some people there are still others like me who they haven't worked for and who are still searching for a different solution.