If this was me, and you do not want a headache later... buy a second hard drive - 5400rpm's are around £30 for a cheap one or £50ish for a cheap 7200RPM and replace it with the one in the laptop. (edit - this assumes that the laptop is not running TPM / Bitlocker or anything where the machine is locked to using a single hard drive)
This would be a easy swap (you may have to change around a caddy and use a few screws...) and it would mean returning the computer identically to how it is when you got it.
You can then get a usb caddy or similar from ebay (or a computer shop) for under £10 and use your new laptop hard drive as a large removable drive!
edit - since you can't change hardware. This is the long way round, but its free and should do what you want... you will need a spare machine, or install then remove.
The difficult way - download the Windows OPK / Deployment tools - http://technet.microsoft.com/en-gb/solutionaccelerators/dd407791.aspx
From there, you need to follow the guides and build a bootable disk with imagex on it... or use any bootable win vista or win 7 disk and have imagex on a usb stick.
The easy way - obtain a iso (technet, MSDN or even from a cd) of Windows 7 or Windows Vista, Any version, 32 or 64 bit.
Copy the entire disk to a usb stick, If you can not find a USB stick big enough, you can leave the huge file in the /sources directory out since we are not actually going to install windows.
Next, you need to get imagex, http://www.tipandtrick.net/2008/imagex-600118000-x86-and-x64-for-windows-server-2008-and-vista-sp1-standalone-download/
Copy that to the usb stick. Also, to make it a lot easier, I just found this (but can not guarantee as never used it, but it looks good) http://www.autoitscript.com/gimagex/
Then, you want to basically capture your hard drive into a .wim file, use the gui or this :
imagex /capture c: c:\images /compress maximum
Full syntax available -
imagex /capture image_path image_file "name" {"description"} {/boot | /check | /compress [type] | /config | /flags "EditionID" | /norpfix | /scroll | /verify}
http://technet.microsoft.com/en-us/library/cc749447%28WS.10%29.aspx
You can leave out the compress bit if you want, but may be worth your while
You are better off actually backing up to a usb hard drive / flash stick / network drive, but if that is a problem - you can just put it on the main hard drive.
After you have taken a backup of the .wim file, do whatever you want with the laptop!
To restore, simply boot into the windows pe environment again and run (done x: as I do not think this can be on the c drive when running).
imagex /apply x:\images\my_image.wim 1 c:\ /verify
Full syntax:
imagex /apply image_file image_numberimage_nameimage_path {/check | /ref | /scroll | /verify}