Its not a problem related to vista vs XP.
Theres something definatly up with your card/hardware.
Could be overheating causing downclocking or possibly a PSU issue.
Sorta.
Skulltrail is based on the X38 chipset, however it offers two phyiscial processor chip capability, somthing with X38 doesnt.
It achives Nvidia SLI through a pair of MCP 100 nvidia chips, which allow SLI without a Nvidia chipset
Unfortunatly HardForums don't seem to take Overclocking as enthusiastically as Xtreme Systems
In particular the guy haveing a smoke while filling his liquid nitrogen is extreamly appleasing http://www.xtremesystems.org/forums/showthread.php?t=171383
Same with any company that allows modded heatsinks...
EVGA, BFG XFX etc...
Probably because if they need to replace the card they dont want to be swapping heatsinks off all the time, Lost heatsinks etc etc...
Saves everyone the bother if you just sent back the Graphics card like it came.
The old stock cooling worked just fine, you would just have to increase the fanspeed to get the temps down a little.
XFX and EVGA both have revised editions of the fan size
... This posts confuddels me..
Have you registered for Stepup??
Or you've already registered and forgot to send it in within a 15 day timeframe??
Or have you registered and in a queue?
Best bet is to email them
Xp or Vista?
Also the 8800GT run at a default fan speed of 29% which doesnt change unless you either flash the bios with the latest manufacters version or download RivaTuner and change the fan speed in there.
Some more infomation on your hardware would be great
Download Drive Cleaner Pro and boot into safe mode through F8 / F6 and run the program to remove Nvidia graphics adaptor.
Restart and install the latest Forceware from Nvidia's site
The HR-03 Plus works on G80 series only.
the HR-03 GT works on the G92 series
http://www.thermalright.com/new_a_page/product_page/vga/hr03gt/faq_vga_cooler_hr03gt.htm
First FAQ gives you the answer