When you install more than 1 Microsft OS, it installs a kind of boot manager for you. You get a list in DOS after it's done all it's hardware checking on starting the pc.
Just before where you normally get the win98 logo when you turn on your pc, there's a list like :
Choose the OS :
1) Win98
2) Win2000

Time remaining : 30 secs
Where the time remaining counts down - during this time you press the up & down arrow keys to select which os to use, if the time remaining figure (which you can change within windows) runs down to 0 - the default OS is picked and loaded.

Most versions of Linux have their own boot loader (is it gnome that's one of them)? Thought I don't know how this works, and I guess the above would still be kept after you've chosen whether to boot into linux or windows.