In the general case, Windows is not very reliable at being booted physically (i.e., not virtually) on a foreign PC after it's been installed. This can be due to many reasons, not the least of which are the following:
- Wrong PC architecture type (32-bit vs. 64-bit)
- AHCI vs. IDE compatibility mode for the SATA controller
- Graphics drivers
- PC-BIOS vs. UEFI bios (the Mac uses EFI so unless your PC used UEFI your Windows can't boot under UEFI)
- Running your OS image on a completely new system will invariably trigger Windows Product Activation (WPA), which will require you to re-activate your copy of Windows anyway, which is mainly what you said you wanted to avoid. Windows will detect "too many" hardware changes (CPU, RAM, motherboard, graphics card, disk, everything) and flag it as requiring re-activation over the Internet or phone.
However, if you are adamant to trying it, you can follow a manual procedure to try to emulate what Boot Camp does but with your own ingredients.
You can use gparted as your re-partitioning tool if you insist on a GUI, but you can also use diskutil from the command line, there are various other ways too. The gist is that you need to non-destructively shrink your Mac OS X partition and then do a low level (block-level) copy of the data blocks from your Windows partition to your Mac HDD. This will be impossible if you can't shrink your HFS+ volume to be smaller than the total size of the incoming NTFS partition.
If you want a detailed step by step guide (note that this guide does NOT tell you how to copy your data over), go here: http://www.tech-recipes.com/rx/9839/how-to-manually-partition-boot-camp-your-mac-for-windows-7/
Note that you can't just type the literal command line options in verbatim from the recipe because you'll need to size your partition appropriately so that the unpartitioned space you create is at least as big as the partition you're inserting into the volume. This means that you must understand the meaning of the commands you are typing into the console rather than just copy and pasting.
If you are not a very patient and technically-oriented user and this sounds daunting, I'd say don't even attempt it. You're going to have to reactivate Windows either way.