I think VMware is your solution here - you'll want ESXi. Although VMware "technically" supports up to 128 machines per physical server, I've seen a demonstration where they had 178 running! So this won't be an issue.
Depending on the performance requirements, here's what you'll want to do:
Get yourself a beefy server. At least dual-quad i7's. Also, go for a configuration of about 96GB RAM. You can halve that if you only need barebones performance, but really don't shoot yourself in the foot and go with too little RAM - it's not that expensive these days. With the i7's running optimized with RAM in triplets, this is an ideal configuration for performance. You can assign each VM 1GB, which should be adequate for your testing environment. Also, you'll want maybe 4 drives in a RAID 10 for performance. You can thin-provision which is nice - assign each machine maybe 20 GB for the install, but it will only take the space needed for the actual files - maybe 2-3GB per server max. Times 100 is about 300GB - go for 4x250GB drives and you'll have the space, performance and redundancy you need.
This server shouldn't cost more than about $7000, depending on your RAM choices. The full 96GB might be a bit more. I'd recommend Dell on this one - they seem to have the best price/performance ratio right now. You won't need extended warranties or business critical support - NBD would be adequate.
The VMware ESXi will be free for you in this case - you don't need anything that would require a license here. Just register for the free license.
The OS license gets a little trickier. I'll leave it to you, but probably you'll want Server 2008 Datacenter, licensed for 2 processors. It's about $4000/processor. This allows an unlimited number of Windows virtual servers to run on the box, and they can be any flavor of windows (Server 2003, 2008, Standard, Enterprise - it doesn't matter) For the Linux boxes it's no issue.
For the machines - the Windows boxes are easy enough. VMware has a cloning tool that works like a disk clone utility. Set up your first server, then prepare it - use Sysprep - then shut it down and clone it. You know, 80 times. Once that's done, fire up each server one at a time, run through the prep tool to rename and whatnot, and you should be good. For the Linux boxes I'm less helpful - you can clone them of course, but after that I'm not much use - I'm a Windows guy.
Total cost here with Licensing is about $15,000. Would that work?