OK, here's the failsafe checklist process. If this does not work (and please make sure you do every step exactly), then it points to a hardware issue...
1) iTunes must be the latest version 10.5, so update it if it is not. Turn off any anti-virus or anti-malware products temporarily.
2) Download the 5.0 firmware for your iPad model to your desktop from this thread -
http://www.ipadforums.net/ipad-hacking/24939-official-apple-ios-firmware-download-links-ipad.html
3) Download TinyUmbrella for your PC from this site -
The Firmware Umbrella - TinyUmbrella
4) Start TU, switch to the "Advanced" tab, UNTICK the box that says "Set Hosts to Cydia on Exit", then exit the TU program (you won't use it any more)
5) Connect your iPad to your PC, and put it into DFU Mode using this guide -
http://www.ipadforums.net/ipad-hacking/27489-dfu-mode-dummies.html
6) Start iTunes, it will recognise your iPad in recovery mode, click OK, the shift-click (Windows) or option-click (OSX) on the "Restore" button and iTunes will prompt you for a firmware file to restore - choose the one you downloaded in step 2.
This is a failsafe method, and the process should complete normally. If it does not, please post the error message iTunes displays this time.