Check your system devices by clicking on Start, Control Panel, and then System. Click on the Hardware tab and then device manager button. Look for any question marks in the list.
After a reformat, allyour Motherboard devices will have to be re-loaded. (Some older MBs are built into XP, but not the newer ones. You didn't mention what wireless device you're using but you probably had to load drivers for that.
If you find question marks, try to download the latest drivers from the manufacturer's website, (most of the main ones have good support for this), or search through your CDs that came with your system. You should have the drivers, (unless you bought the system second hand).
Hope this helps...