Drive indexing is usually the culprit - AV software peforming a complete scan will produce the same results but it's usually easy to check that.

click here

Leave the PC on and unattended for a few hours - set screensaver/power management to allow the machine to stay on for that time, the indexer should finish by that time.

First the "free" AV which is only in beta yet, due to be available around October,
click here

I can only relate my experiences. I too suffered with "disc thrashing" for two years with Vista and spent ages reading up on all the tweaks etc that get mentioned, disable this or that, turn off such and such. Nothing made much difference. I thought it was "normal"
It's useful to put the "CPU meter and RAM" useage meter onto Vista's sidebar. Web browsing would cause a lot of HDD activity.
The long and short of it is that Vista alone isn't to blame.
Just out of interest how long does your PC take to boot from cold. I was getting towards 2.5 to 3 minutes, now it's consistently around 1'12" full Aero and sidebar.

On my notebook (3yr old, 2gb ram, AMD Turion 64 mobile processor) following a clean Vista install and getting it fully updated there is "zero" HDD activity. At this point there was no AV product installed. RAM useage around 27% CPU 1% when idling. All Vista's settings at default, nothing tweaked, not even CC installed. Task manager currently shows 50 processes running. At this point I installed the MS beta AV product. After years with paid for paid for packages I was absolutely amazed that this didn't noticeably impact performance. In the last 10 minutes the HDD light has never more than blinked dimly every couple of seconds. Honest !
Have you opened Windows Defender and looked in software explorer to see what start up programs are running. I have anything that isn't Vista related disabled (applications and programs I have installed).
If you open "Performance and reliability" monitor what is your Disk useage shown as ? Below the graphs are drop down menus, the one marked DISK will say to the right in % and kb/mb per second what it's running at. On this web page mine is showing mostly zero with an odd jump to around 12 to 16kb/s.
So what I am getting at is what's achievable... and all this with Vista at default settings. The thing is I haven't done anything to get it like this... this is Vista on it's own.

In performance monitor above if you look at the drop down list under "disk" whatever is using all the resources should be shown at the top of the list.
For it to be at 98% something is canning the HDD.
Was this a clean install of Vista you did, or an upgrade, or preinstalled when you got the PC ?