The same thing happened to me while I was uninstalling a few programs and I just did a system restore; volume came right back!
Start --> All Programs --> Accessories --> System Tools --> System Restore
When you get there, either
(A) Pick a day and time YOU KNOW your volume was working correctly and restore from there (For example, you know your volume was working on 9/3/08 at 10:54 PM but it disappeared at 11:01 PM. So, you would want to restore your computer to 9/3/08 at 10:51 PM because YOU KNOW your volume was working then)
(B) Pick one of the activities you recently used when you KNOW your volume was working correctly and restore from there.
A system restore basically takes your computer to the past and restarts it (with all your settings) from the past time of your choice--> you are literally "UNDO"ING anything you did on your computer AFTER your volume broke. Therefore, you may have to re download, install, etc. things that you completed after the date you are going to restore from.
Good Luck and I hope it works for you!