While I agree that using a dedicated machine for one's Linux adventures is a safer option, I'm uneasy with the implications. Essentially, it's saying "if you want to try out Linux, a free-as-in-beer operating system, it's best to buy another computer for this." Even at $100 or less, it's still money, to say nothing of finding the room for another computer. In the time it would take to shop for and set up a used computer, a user could learn how to dual boot with Windows safely.
I've been using Linux since maybe 1999. I've never had a dedicated Linux computer and have always dual booted with Windows. I've *never* trashed a Windows partition or lost any data while using Linux. I am not an expert in any way. My approach has always been to learn what I needed to know in order to do what I wanted to do and not go beyond that. "Use it or lose it" is true when it comes to computers. If you learn something you won't be using, chances are very high that you'll forget it. I learned how to partition a drive long before I ever heard of Linux. I've always been well backed up, going back to the PC Tools Backup on floppy disks in the DOS days. With good backups you need never fear losing data or trashing a system. If things go bad, simply restore your system. Image files are particularly desirable--essential, really, for easy recovery in case things go wrong. Seagate and Maxtor drives come with an OEM version of Acronis TrueImage, which I use for system backups.
A few things I'd recommend for safely having Windows and Linux on the same computer or same drive:
** Don't put a Linux boot loader in the MBR. Use LILO on a floppy disk if your computer has a floppy drive. If not and you're using XP, put the MBR in the boot sector of your Linux partition and use the XP boot loader to chainload to LILO on the Linux partition. Or simply boot with the VectorLinux installation CD. It's not much extra work to boot with the VL CD.
** Don't autoload a Windows partition, particularly not an NTFS partition, until you know what you're doing.
** Don't autoload partitions for other Linux installations until you know what you're doing. You can make a mess of ownership and permissions if UserIDs and GroupIDs don't match up between installations.
** For maximum safety, act as if the Windows partitions and other Linux partitions don't exist. If you don't mount these "alien" drives, your VectorLinux won't know they exist for all practical purposes.
These aren't "forever" rules, but until you know what you're doing they'll keep your Windows data safe from newbie mistakes.