before installing PT9 be sure to READ THE DIRECTIONS. Install your updated drivers FIRST.
Driver Downloads and Updates for Pro Tools 9 and 8
Before installing do at least most of the W7 opts.
Avid Windows 7 Optimizations Video Playlist - YouTube
Windows 7 Optimizations
And you should update to 9.0.2 as well, which is free for PT9 owners.
Pro Tools 9.0.2 Update