Windows 7: Starting out with VM

I'm new to VMs and want to start learning by using (safely).
I'm thinking of VirtualBox or VMWARE for running UBUNTU and legacy DOS.
Any recommendations would be welcome.
Also thinking of installing on an external HDD to start with - is that possible?

I would suggest either VMware Player or Workstation. As far as an external HDD, you could install your VMs on there however, the performance penalty in my view is not worth it as they usually run slower than traditional hard drives and are usually USB 2, which is not all that fast for this.

i currently run Ubuntu Linux, WinXP, Win3.11 and DOS6.22 on my Windows 7 computer using VirtualBox with all my Guests stored on my external HD through USB 2. I have had no problems, and I certainly dont have any speed problems. Good luck, and dont hesitate to ask questions.

Thanks for the responses.
There seemed to quite a few negative comments on VirtualBox a while back. But what you've got going Pusspa looks good.

Another few VM novice question if I may:
(1) Am I right in assuming that the virtual environments are in effect applications running on top of Windows 7.
(2) Can they interfere with key windows 7 boot parameters that exist in the MBR (the real one on the first sector) or the BCD store.
(3) A little off topic... where can you get legacy OSs like XP and DOS6 when you don't seem to be able to buy them anymore.

Yes, it's essentially an application (guest) running on top of Windows 7 (host). It's isolated that it won't affect your primary system (in this case, Windows 7) in any way during bootup. You can try ebay for older software such as XP/DOS 6, although curious to why you would want to do that.

Important thing is to have enough memory on your computer, so you can evenly distribute it.

Thanks Slam
For both XP and DOS the interest is legacy applications.
For DOS I have a Fortran compiler (16 bit) which cost ~$1000 in it's day and wrote many simulations using it. Although I have modern 32 bit compilers the old one has a nostalgic value.

The main use of the VM will be for running linux and other applications in a "safe" environment and be able to move back & forward across OSs.

Sounds like VM would definitely be the way to go for all your listed needs.