Windows 7: Slow as can be - Low Resource usage

Question

My main PC is a Dell from 2009 with Win7 SP1 that was originally a Vista machine. I installed a second HDD with Win7 on it and set Vista as the second drive. Its running with an Intel Core Q6600 quad-core processor @ 2.4Ghz with 4 GB Ram. Never had any issues with slugishness in the past really. Just the random freeze up here and there.

A few days ago I performed a hard reset after the computer froze up on me while trying to interact with a video on Youtube. Every restart to Win7 since has resulted in serious slugishness to the point where the computer was almost completely unresponsive.

I booted in Vista to perform a Disk Defrag and performed Check Disk to the Win7 HDD to automatically repair any issues that it found. I'll work on grabbing the report from that Check Disk here shortly but it will most likely take a while considering the slugishness of the computer.

I've managed to boot to Win7 and it is now going through periods of quick response and periods of effectively no response. During those periods of slugishness, Task Manager says there is constantly < 5% CPU usage and around 45% memory usage.

I'm beyond frustrated and I'm not sure what to do here...

Does anybody have any advice/solutions?

Edit: As per techie's suggestion, I've checked out the Resource Monitor.

During the last period of slugishness, the Disk shows 100% as Highest Active Time with a speed of 1 Kb/sec for the entire time I had the Resource Monitor open while the machine was running slugishly. Currently, everything is running just fine but I expect it will slow down soon...

Second Edit: During times of slugishness the disk I/O is always less than or around 1 MB/sec with a highest active time of 100%. There are very few ( < 10 ) hard faults/sec with around 50 % memory in use with the CPU < 5%. Also, it tends to send anything that I try to interact with to a state of "Not responding" for a good minute or so during these periods.