I had mine a similar length of time and can definately relate, I had left click or right click issues constantly. I ordered a new one of the same model (although it turned out to be a 2007 mode rather than my original 2006 model) and I was actually suprised to find how different it was to click the button, a louder and more established click and needless to say no more issues. So perhaps, if you have a retail store that displays them etc. give it a click and see if it feels more pronounced!
I think whats happening is, the button inside is getting gummed up or worn down, I wouldn't recommend it but I found a guide for another mouse and applying some WD40:
http://forums.hardwarezone.com.sg/showthread.php?t=1793115
Not specifically a Windows 7 issues though