Daz Studio does the same thing..refuses to let go after I've had it running any length of time.  
When my pc starts running slow the first thing i do is look at processes. 
Internet Explorer used to hang on too...but since I switched to IE 7 I haven't had any probs with it.

 

“For those who believe, no proof is necessary. For those who don't believe, no proof is possible.”

[Stuart Chase]