Put in a Windows CD then start it up, hopefully it will boot off of the CD. If not you'll have to change the boot sequence in the bios which will be a little more work.
http://www.mayoclinic.org/tests-procedures/tilt-table-test/basics/definition/prc-20019879
Get a tilt table test done, I had the same problem that happened a few years back and it turns out I have syncope. Doctor said that anytime I felt like that to sit on a chair, cross my legs and lean down...