You can manually force BitComet to use less ram by decreasing the Disc cache:
Options>Advanced->Disk cache.
But that won't finish the problem if BitComet does not close.
You DON'T need to stop tasks before leaving BitComet, bitcomet will stop them, but it will take time.
The bigger you cache, the more data will be stored in the RAM before writting it to the Hard Disk, and the more time it will take to finally close, since BitComet tries to prevet file fragmentation by making files as much contiguous as possbile..
But, let's face the truth. The real problem here is Vista.
You should have stayed with XP until they released Windows 7. Vista sucks almost as much as Windows Me. Sorry.
Head in the clouds. Feet on the ground.