I do not know why this is happening, as there can be several causes, but there are somme common-sense places to start:
It could be your your usb
drivers have an issue, and by going into device manager, you can check and see if there are any hardware
driver errors.
Equally possible, it can be the
drivers for your USB devices, such as a non-Windows 7 friendly mouse or keyboard
driver, again, pretty easy to check ou.
Also, it may well be that either your mouse or keyboard may be faulty, holding up your system. The easy way to check this is to simply try a different keyboard and mouse!
Last, but not least, it could be a bad usb port, or less likely, even a setting in your bios. The bios of course is easy enough to check out, but running down a bad usb port can actually be tricky...and yes, they can go bad!
The first thing that I would try, to eliminate the obvious, would be to try a different mouse and keyboard (and anything else that is plugged into your USB ports, by the way! For example, although your mouse/keyboard is slow to initialize, it COULD be an external USB hard drive causing an issue)