Feret: Here's some tips for your lock-up problems. It would appear that you have interrupt conflicts, probably from when you installed the USB ports. These are now using the same interrupts as one or more other devices. If you can, go to the System Devices in the Control Panel (you'll need Administrator privledges to do this) and have a look at the device profiles. This should give you info on which devices are conflicting. You can then change the interrupt settings so that there are no conflicts.
If this doesn't work, the only other option I can suggest is copying off files you wish to keep (pictures, documents etc) and re-installing. Not the sort of thing you want to hear I know, but...