Actually, you
don't need a PC to slipstream to a disk image, but you may need one to burn CD for installing on Boot Camp. I haven't tried that (since I haven't actually installed Boot Camp yet so I don't know if burning from the iMac will work), but it should be easy to borrow the use of a PC for a few minutes if that's needed.
I slipstreamed SP2 onto an SP1 windows by doing this:
1 Installed Parallels.
2 Installed SP1 Windows VM.
3 Copied Win XP disk onto VM hard drive.
4 Downloaded nlite from
www.nliteos.com onto VM.
5 Downloaded SP2 installer from Microsoft onto VM. I also downloaded as many of the standalone hotfixes and security updates as I could find.
6 Started nlite and followed the steps for slipstreaming SP2 into the directory created in step 3. Also added the hotfixes and security update.
7 Burned a bootable ISO.
8 Used ISO to install SP2 WinXP on another VM to make sure it worked. It did!
Steps 3 to 8 are exactly the same as the easiest way to slipstream on a PC. The only step I didn't do was to try to burn the ISO to a CD. I might do that later if I decide to install Boot Camp, but for Parallels, it's not needed.
Edit: I should have made it clear though, it's not legal to have two Windows installs using one licence either in Parallel VMs or Parallels and Boot Camp. You need to remove the first install if you do what I did.
Also, you do not need to activate Windows immediately, so if you screw things up you're not using up an activation key. You have 30 days to activate it, so you can (legally I hope) borrow a Windows install disk and play about with it for up to 30 days before you have to get a licence. This way, you can determine if running Windows is for you.
Edit 2: Thinking about this, I see no reason why using Disk Utility on the iMac to burn an ISO built in a VM wouldn't work. The work (described in the slipstreaming guide you link to) that's involved in creating the boot disk is done by nlite. Moving the ISO from the VM to the iMac is easy with a SMB share.