Did you...
verify the CPU is seated properly?
verify the RAM is seated properly?
verify all cables are plugged in securely?
verify cables are not pinched between solder points on board/chips and casing?
verify you didn't scratch the board/s removing screws or putting back in?
verify you used all screws removed?
try putting the original CPU back in?
try putting the original RAM back in?
A little paste off of the primary chip shouldn't be an issue unless it was making contact with pins or other metal leads. The tops of the chips usually don't have this.
From Arctic Silver website
Not Electrically Conductive:
Arctic Silver 5 was formulated to conduct heat, not electricity.
(While much safer than electrically conductive silver and copper greases, Arctic Silver 5 should be kept away from electrical traces, pins, and leads. While it is not electrically conductive, the compound is very slightly capacitive and could potentially cause problems if it bridges two close-proximity electrical paths.)
Hopefully this can get you pointed in the right direction.
Outlaw