Stumped Random Mouse Error?

i'm new to these forums. For the last week, i've been using an older HP Pavilon system, 933mhz with 512mb ram. My issue is this, Occasionally, when I turn on the machine, I can left click the background no problem, but if I try to click on the start menu, it's as if the left-click on my mouse doesn't work. I've found a remedy to this problem to be to alt-tab, even if no other programs are open at the time. If I alt-tab, i'm 99.9% of the time able to left click the menu bar. Now, this wouldn't be a major problem, however I have the same issue when I alt-tab through webpages, or if I alt-tab out of a computer game. Also, I play World of Warcraft, and every so often the same thing happens, however to anyone who may play the game- I am able to use the leftclick on the mainscreen, as in, to pan around my character, but if I attempt to open my bag slots, click on a monster or any of my skills, nothing happens. I've tried replacing my mouse, no good. I've tried differnt ram, same problem. I've tried a new hard drive, no good. I've FFR'd both hard drives, reinstalled windows from scratch, and STILL the problem is there. Does anyone have the slightest idea of what could be causing this?