You can easily dual boot ubuntu along with Windows on the same or separate drive easily. Depending on version like either XP or Vista/7 the boot loader used will be decided on.
For XP with Linux that would be Grub, wubi, Lilo for the Linux boot loaders. Some time back ubuntu had a 3.5" boot floppy where you simply selected a partition and the OS on it would load right up!
There was no need to edit this or copy that over from ... there. WinGrub and SuperGrub are two cds seen now.
Vista isn't is all bad and resource hungry as it is made out to be if you simply follow a few performance tips and run a more upto date system. That's what catches everyone at first running the old 512mb-1gb 2003-4 deals while the 2007 OS is based around 2006-7 1-2gb systems with faster cpus and larger hard drives.
Stable yes! Far better then all the crashes typically seen in XP until finally seeing SP3 and some 1,000+ fixes in that. But so far 7 blows right past the two previous versions there! You won't need any 4-8gb dimmed up pc to run this one! XP, Vista see yaa...