I usually just use an Amazon EC2 instance for Windows browser testing. I've got an essentially bare Win machine image (just the common browsers). I fire it up and then use Remote Desktop to connect to it. Works pretty well (other than, as you say, being limited to Remote Desktop/VNC-like speeds).
Another big advantage of doing this over using a local VM is that when you have to run the inevitable 12 terabyte software update (with six reboots) after not using it for a while, the updates download at Amazon data center speed, not your local connection speed.
Just make sure to save the updated machine image afterward. :-)
If you have a copy of Windows (legally or one you are willing to use anyway), I find installing it on a VirtualBox VM more convenient (and entirely free) than Remote Desktop to an EC2. And faster.
The alternatives discussed here are for when you don't have and don't want to pay for a Windows license, and are unwilling to break the law and pirate it as part of your job.
My (completely legal) copy of Windows decides that it's not legit every time I upgrade VirtualBox, and sometimes at random times in between. Each time I have to dig out the disk and go through the authentication BS.
If I actually used it for anything other than occasional browser testing it might be worthwhile to jump through those hoops, but I don't. Not to mention that (since I only need it once in a great while) each time I use it I have to go through downloading a zillion megabytes of software updates and reboot multiple times.
Also, I don't have to keep it around on my (always full) main drive or run it off a slow external drive.
The two cents an hour cost for the AMI is well worth avoiding the hassle, for me. Your use case may well be different.
