There's a few companies that make "pico-ITX" boards that run x86 processors [1]. Mostly low powered chips like Atom, but you can get Core 2 Duo boards if you've got the cash. Prices started around $300 last time I checked.

[1] http://www2.warwick.ac.uk/fac/sci/eng/meng/wmr/projects/uav/...

There's tons of book-sized systems available, typically with lower-end AMD or Intel chips. Throw in a DDR2/3 SODIMM and a SATA drive and you're good to go. These start around $100.

Example: http://www.newegg.com/Product/Product.aspx?Item=N82E16856205...

Intel are just about to release the "NUC" Next Unit of Computing. Looks pretty reasonable, but significantly more powerful and thus expensive than a PI.

Not sure about windows, but VIA is making something somewhat similar but with a x86 processor: http://apc.io