I'm running the 32-bit version of Vista Home Premium and I know about the limits of 32-bit memory addressing and the situation where Windows can only see around 3.5 GB due to Windows keeping around 512 MB in reserve for the operating system.
When I first installed Vista, it was showing around 3 GB in the Windows Task Manager.
There's also an additional 512 MB being used in my graphics card leaving around 512 MB for the reserve that I've just mentioned.
Now there's only around 2.5 GB showing in Windows Task Manager (below)
"I must confess I don't know why yours should be showing less than it used to unless you have fitted any recent card/s that contain RAM?" ---------------------------------------------------- The only thing that I've fitted to the PC (apart from a DVD writer) is click here but it doesn't come with any memory as far as I know.
It must be the cause, as come to think of it, the missing 512 MB only came about since fitting it a few days ago!
Some of the Creative cards have onboard memory but not the extreme audio. It does reserve memory for Soundfonts - the samples used for midi playback - but this is relatively small (in the order of a few Mb). It will also be using IO space above 2Gb which is more likely to be the reason your memory is suffering.
In the second screenshot, my graphics card seems to be grabbing 1 GB of system RAM but that still doesn't explain what happened to the missing 512 MB i.e. if you add up the 2.5 GB showing in WTM and add it to the 1 GB that my graphics card claims, it still leaves 512 MB unaccounted for.
The 2.5 GB limit is also shown in the 3rd screenshot on the NOD32 status screen.