dual boot - My partition table is facing limitations; having problems creating more partitions

I first noticed an issue when trying to install Linux Mint 14 as a third OS alongside Ubuntu 12.10 and Windows 7 - I was unable to create another partition to install Mint to.

Poking around, I realised that I had reached the limit of primary partitions: (from left to right of the table) 1) a ~100 MB primary partition that I meant to use for storing Grub files but never got down to, 2) a 25 GB extended partition, with the entire partition containing an ext4 Ubuntu partition, 3) a ~513 GB partition containing Windows 7, 4) some ~50 GB of unallocated space, and finally 5) a 4 GB swap partition.

I decided to wipe off the first ~100 MB partition, which I didn't need anyway. This brought me to a dilemma, however: as Ubuntu, during initial installation, had created an extended partition for itself, which meant that I could not create any more. Having an extended partition for Ubuntu is virtually useless as there is only enough space to contain Ubuntu itself, and the benefits of extended are worthless here. And if I go ahead and install Mint now, I would face the same issue of too many primary partitions in the future should I want to create any more.

What I'd really like to do is this: convert the current extended partition into a normal primary partition, and create a new extended partition in the unallocated space for all future partitions. Is this possible, and if so how? Thanks in advance!

My FixParts program can convert from primary to logical and vice-versa, albeit with some caveats concerning what's legal in terms of MBR partition layouts. If I understand your layout and goals, FixParts should do what you need. This is likely to be safer than using fdisk or parted, since there's less room for user error. I'd recommend also converting the swap partition to a logical while you're at it, since that will reduce your primary partition count to 3, giving your more options should you ever need another primary partition. If you convert your swap space to a logical partition, you'll then need to use GParted to resize the extended partition that FixParts will create around the swap partition. It's best to do all of this from an emergency disc.