Not recommended. Take the steps to do it right to avoid future problems.
Follow this closely: Partition : Recover Space Used by an Older OS
You will have to add a first step using Partition Wizard CD to rightclick Windows 7 partition to Modify>Convert to Primary, OK, so you can proceed to Step One to Modify>Set to Active.
Pay special attention to Step One explanation on marking Windows 7 partition active, so that after the Partition Wizard CD steps are completed you remember to boot the Windows 7 DVD or Repair CD to run Startup Repair up to 3 separate times to write the System boot files to Windows 7.
Backup your files and a Windows 7 backup image so you can start over if necessary.
Thanks for the response. Before you got back to me, I did the following:
I moved the boot files from HDD to a temporary folder and after a cannot-boot error found out that Windows is actually still using the boot files from the XP installation. I then used a Linux live-cd to move the boot files back to their proper place on HDD, I ended up deleting all the files on HDD except the boot files and the contents of the Boot folder (which didn't show up in Windows).
After that, I resized the HDD partition down to 5 GB (smallest I could in gParted), and I created a new partition for my personal files in the unallocated space between HDD and HDD7. For the record, I renamed HDD to BOOT, as it is now basically a partition for booting.
If anyone else stumbles across this, this is my fix that worked for me, although following the steps Greg posted is definitely the cleaner way to do things.