Your bags should be checked through.

I'd just go out the main exit from customs, cross the road to the T5 bus station and take the No. 71 or 77 bus straight to the High Street in Windsor opposite the castle entrance, which takes around 40 min. and around £2 or £3 each way IIRC. If you don't want to bother with the castle, there's an excellent pedestrian precinct right at the bus stop, with plenty of shops, cafes, pubs etc. to help kill a few hours. Beats the hell out of staying airside IMO.