I've heard good things about VMware. Here at work, however, we use Virtual PC, http://www.connectix.com
. The biggest con is that Microsoft just bought out the company (jealous of the technology or concerned about OS licensing on "temporary" images, dunno which). You can only get a trial evaluation version at the moment until MS produces their own b'tard.
VPC does run on HT hosts and does NOT reproduce HT in the VPC image. It emulates a single CPU, a generic ethernet card, and a very generic (S3?) graphics card. It is GREAT for non-hardware specific testing and troubleshooting. We use it here for testing software installations and troubleshooting the same. Again, we mainly care about the different OSs and not the hardware in our environment.
The main complaint I ever saw in Connectix forums is that you can't emulate SCSI devices (it emulates ATA\IDE HDD). Not a prob for us, unless you want to Ghost a SCSI-based PC to a VPC image. (You can Ghost a PC to a VPC, but not if it's SCSI.)