Virtual Machine can make it go slower but beyond that, no. We play around on virtual machines in my class all the time since we can actually install things in them as opposed to the "no installing" thing we normally have on the computers.
Just to clarify, a virtual machine will only slow down the host machine when it is powered on and active. As reider said, it cannot have any other effect.
So the guest machine won't slow down the host unless it's actually powered on... ok could the guest machine be slowed down by the host?
Easy answer: PC emulator. It reads the instructions from programs using software instead of hardware. So you are 'creating' a piece of hardware using software, where creating means virtual.
Easier answer: Virtual machines let you run another OS in a window on your original OS without actually installing it
Lets put it this way... You're running a Mac.. You need to use a program you can only use on Windows.. You run a for virtual machines, how about Virtual Box. It simulates running Windows in a window on your Mac. Allowing you to use windows programs and services. It'll slow it down because you set how much base RAM the virtual machine can run. I think the recommended base amount for some is 192mb. Which is pretty slow. If you have a 3/4gb machine then I think you would be able to get away using 1gb of base Ram, or a little more if you feel like it. The best way is to experiment.
VM's are very useful for trying out programs that your not too sure of. or to do anything else that might have a risk of getting a virus from.
