Sure. I'd suggest a backup hard drive for starters. In particular I'd recommend a 2.5" USB or Firewire enclosure with a 5400rpm drive since that can be powered by a laptop or desktop without the need for an additional A/C adapter. Note that Intel-based Macs can boot from a USB or Firewire connection while older PowerPC Macs can only boot from Firewire drives. I recommend the Firewire/USB2 enclosure because Macs like Firewire better while PCs like USB, especially for backup apps. As far as software goes, for Mac there is SuperDuper or CarbonCopyCloner. SuperDuper is a more professional app (free version & pay-for version); if you are going to use it in a business environment I would recommend that one.
For Windows, it depends on what you go with. Boot Camp gives you the real Windows experience since you're dual-booting, but the software is still in beta. Parallels is more than good enough for nearly everything and is out of beta now. The only big thing you can't do on Parallels is play 3D video games. If you decided to go with Boot Camp, you will need a BartPE disc with the DriveImageXML plugin. It is the only system that I've researched that has worked for cloning Boot Camp partitions. However, it's not a simple procedure and not something you'd want field users doing themselves in event of a catastrophe (again, Boot Camp is still in beta).
I would highly recommend going with Parallels. Backup is a simple procedure; you simply need to copy the operating system folder to a backup drive. Inside the folder is a configuration file and a virtual hard drive file. One of the neat features in Parallels is the way that virtual hard drive is handled. Let's say you want to make a 60gb Windows virtual hard drive under Parallels: they give you the choice of a set or expanding hard drive. If it's set it means the file is 60gb regardless of what's in it. The expanding hard drive is much better because even though the virtual hard drive is 60gb, in reality the file is only as big as what is in the virtual hard drive. So if you installs Windows, apps, and files and it only comes up to 8 gigs, then your virtual hard drive is only 8 gigs instead of 60 gigs. The end result is that you have a much smaller and more portable file to deal with, making backups easier. Oh and the latest release of Parallels (from March) has some pretty neat features; check out my mini review here:
http://www.mac-forums.com/forums/showthread.php?t=63278
You can store the image on any drive or disc that has enough space for the current file size. Thumb drives are good, but big ones are expensive and small (16gb USB stick is over $100). DVDs are good but slow and also small (between 4.7 and 8.5 gigs each). You can buy rewritables, but again there's that speed issue. In my opinion, if you need portability the best backup device is a 2.5" hard drive in a Firewire/USB2 case. You can do a good backup drive system for around $100, here are some sample links:
$30 Firewire/USB2 case
$63 100gb 5400rpm hard drive
If you use SuperDuper, you can do a super-simple backup system. Assuming you keep the Parallels virtual machines on the primary hard drive of your systems, you can simply do a regular clone-to-file using SuperDuper. This will clone your OS X installation as well as all of the apps and files on the hard drive,
including Parallels and your virtual machines. This gives you a full system backup of both Mac and Windows. It takes awhile, but you can do it on a lunchbreak or when you're asleep. Again SuperDuper is a good, commercially-supported application and Parallels isn't in beta like Boot Camp is. Another option, even better, is to clone the hard drive to the backup drive using SuperDuper. Macs have the ability to boot from a Firewire drive in addition to the internal IDE or SATA hard drive (note that they cannot boot from a USB hard drive). Here is a link to SuperDuper:
http://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html
Bottom line: For the simplest solution, use Parallels for Windows. For backup software, get SuperDuper. For backup hardware, get a Firewire hard drive that is equal to or bigger in size than the internal hard drive in your Mac. This will allow you to use SuperDuper to clone a bootable backup of your internal hard drive onto your external hard drive. Parallels will be copied over in that process, so you won't have to worry about backing up your Windows installation. In the event of a primary hard drive failure, you could simply boot up using your external Firewire drive as normal. SuperDuper also includes a feature called Smart Update that allows you to update an existing clone, so you don't have to completely clone the primary hard drive every time you want to make a full backup. Let me know if you have any questions.