SpeedswitchXP will not work on the 3790 unless SP2 is installed. This was one of the reasons we found out the cpu on the 3790 was not throttling up and down. We downloaded Mobmeter and noted the cpu speed was not fluctuating, but was stuck at either 600mhz or 2ghz (depending on where we had the BIOS set). So, we tried SpeedswitchXP, only to discover, it would have no effect on the cpu speed, either. Playing with Windows' own power settings would fail as well. That's how this whole issue evolved. It was only after Luke took the issue to Sager (several times) that the solution (posted by Luke above) was discovered. As to why SP2 is required for this to work properly . . . only The Shadow knows.
I wil say that, now that I've had SP2 running well on my 3790 for several weeks, I did try to run SpeedswitchXP again. It seems to work fine, now.