The system is slowing to a crawl though.
That's only (indirectly) related to RAM usage. The PC being slow is down to (basically) three things: (1) lots of CPU usage; (2) things needing to be paged-in when they're needed; (3) programs blocking on synchronous I/O.
Only (2) is RAM-related. And, yes, you might be right that something's taking too much RAM and squeezing other programs (or itself) out, but you should check that assumption first.
All of these values are in Task Manager, as optional columns in the Details tab. However, you might find Process Explorer easier to read.
Or you can poke around in Performance Monitor. It's been a while since I've had to look into any of this stuff, so I don't remember exactly which metrics you need to be looking at.