CPU Stuck at Low Speed (75-78 Mhz)

Hello, I am using an Alienware 15 laptop that is about two and a half years old, and it is having a very odd problem. It's taken me a lot of time to pinpoint what is wrong with this but the other day I finally pinpointed something, and I've been unable to find a solution to it. For awhile now my computer has been slow at a few things, primarily games. Games that used to run just fine are now extremely sluggish, which of course is unacceptable for a system meant for playing games. After a lot of trial and error and running checks and diagnostics that all came back just fine, I finally noticed that, even though I have everything on the highest performance setting and everything along those lines, the cpu refuses to go faster than 78Mhz even though the computer knows it can go up to 2.9Ghz, and the usage percentage never goes beyond around 27%, in fact it flatlines there if put under stress. The only thing I've managed to make have any effect on it is disabling SpeedStep in the BiOS, which made it go up to exactly 80Mhz, and also made 80Mhz the maximum instead of 2.9Ghz, which obviously does not much at all to fix the problem, heh. The only other problem with the computer is that the battery has conked out, but looking that up, having no battery power should do nothing to affect the computer's performance.

So, anyway, I am stumped on this and have no idea how to go about fixing this, so any help is very much appreciated!