I recently upgraded my PC. Everything was fine, working perfectly. Today I upgraded my last component which was my video card. I went from an ATI HD4670 in crossfire to one single ATI 5850. I hooked up all the power cables as required, and when I went to boot up i heard one single beep followed by my LED on the front of the case and on the mobo flashing quickly.
When I unplug it fades away, and returns when i plug the power supply back in. This is my current system.
AMD Phenom II X4 965 BE
MSI 790FX-GD70 Mobo
GeminII S RR-CCH-PBU1-GP Cooler
8 Gigs (4 by 2gb) G. Skill DDR3 1600
Intel X-25M G2 SSD
WD 750 gig 7200 rpm HD
Thermaltake 750w PSU
Windows 7 Home
ATI HD 5850
Follow the checklist in the "boot problems" sticky. Its almost always a simple things you forget. The secondary motherboard power cable, CPU fan power plug, one of the front panel cables fell out or is on wrong, etc.