Open System from Control Panel. Click on the Hardware tab, then in the System Properties window that pops up clik on Device Manager (middle right)
Expand the Display Adapters line by clicking on the plus sign to the left of the line.
If there are ay items under this expanded view showing a yellow question mark or an exclamation mark these will be the devices that are showing as "unknown" when you start up.
Right click on one of the items with a question mark and select Update Driver. Suggest you use the original installation CD for your Radeon card as the driver will be on it somewhere.
If the question marks are not under Display Adapter it would appear that your Windows update has knocked out a driver or replaced a file with a later version that the particular bit of hardware does not like. Same applies, reinstall the driver from the original CD via control panel/device manager.
Post back here with the results to let us know:
a) how you got on and b) which items were coming up as "unknown"