I suggest updating "GB" in your footer, also we also need to know if you are running 32 bit windows or 64 bit windows, and indeed what version of windows. Also Sonar 64 bit or 32 bit?
32 bit only has 4Gb of addressable memory.
Also please check:
Scroll down to "Do You Have Enough Memory?"
http://blogs.technet.com/b/markrussinovich/archive/2008/07/21/3092070.aspx Sonar should be giving you some idea of how much memory you are using too.
With a lack of memory you could be swapping out too much to hard drive, something to check anyway.
Probably barking up the wrong tree here but worth a check.
Oh and please defrag your hard drive.
Cheers..