Slow During And After Startup - Svchost.exe?

For a quick run down of specs, I'm running Home premium 64-bit on an Advent Roma 4001, Intel Dual Core T4400, 4GB RAM and 500GB HD.

It's during the log-in screen that it tends to hang, on both the "Please wait..." and "Welcome" messages it takes between 30 seconds - 2 minutes, but the main problem occurs after logging in. The CPU usage, apart from System Idle Process (of course) is pretty much zero, and the RAM usage is down at about 30MB. Everything seems responsive, however, when I attempt to open a program or process I'm getting incredible lag, the type of unresponsiveness that would tempt you to do a hard reset.

The strange thing is it doesn't actually appear to be unresponsive. The programs all load given time and, for all running slowly for an amount of time, still run. What I have noticed is that svchost.exe seems to kick in at an undefined time and puts the RAM usage up to about 62,000K, with the CPU usage still being very low. When terminating the process, the theme will lose it's transparenecy and revert to a white theme, with processes continuing to run extremely slowly, until a few minutes pass, the windows 7 theme (with transparency) kicks in again and everything loads up immediately. After that, everything's fine - lightning fast.

I've tried chkdsk, messing around with msconfig and various other things to no avail. Am I missing something here? I'm stumped. I think given the problems I've been having aside from this (booting) I'm tempted to put it down to a faulty CPU or Harddrive (the 500GB size supports that probability, I think).