I've been wanting to jump into the Mac scene for awhile now, and these new iMacs are exactly the excuse I've been needing to force myself to do it. An iMac with a dual-core Intel chip? Man, two years ago that would have been unthinkable.
In addition to HijackThis, I use CWShredder, if only because it makes life easier and automatically removes a lot of the problem programs. HijackThis only gives you a big long list and lets you pick and choose what to remove.