I bought a hp dc7900 montherboard, and it's a socket 775, I threw my core 2 quad q6600 in it and when I went to put the heatsink on, it did not fit, the holes on the motherboard are spread far apart. What should I do?
That motherboard is designed to be used in an HP case, with a cooler meant for that case and motherboard. You can't just put any heatsink on it without a little handywork.
As with most proprietary PC builds you have to use there heatsink. Well you can adapt it to fit I have done it on a couple builds but do not remember off hand what we had to do to get it to work it has been awhile. I will check with my friend and see if I can remember what we did as his son is still using that rig.
Some heatsinks have extra wide holes on their mounting hardware (my Hyper 212 did) that might fit. You could measure from the hole centers and ask their support if it will work.