Macbook deadlock after updated to Mac os x 10.6.6

Three days before I update my macbook to 10.6.6, after that whenever I login into my account, it will hung up three, a 

Spinning wait cursor will appear on all programs that I start, and nothing can be inputed into these program.

 

I search through the web and find that pressing key combination Option+Command+p+r on booting can clear some configuration ram, and return the mac to its default setting.

 

It works anyway, but I still dont understand why.