Assuming that you're using Windows 2000 or later, try the following.
Go to 'My Computer', right click on the icon and select 'Manage'.
Select the Device Manager and look on the right-hand side. Any malfunctioning devices will appear with little yellow exclamation mark icons. For anything that's camera or USB related, right click on the icon and select 'Uninstall'.
Once that's sorted, right-click on the computer icon at the top of the device list and select 'Scan for hardware changes'. This will begin the reinstallation process and will prompt you for disks if necessary. If the camera was working correctly before, just let Windows follow the process automatically, but if it never worked properly, tell it that you have the disk and insert it at the appropriate time. (Assuming that you have the disk!)
One other thing, if you recently plugged in a high power device like a hard drive, without external power, Windows may have detected a power surge and disabled the port, in which case it might not come back without a reboot.
