My drive has a 400Gb shared Data partition in the middle and a 30Gb Windows 7 partition at the very back. When I got into cfdisk to install Arch, it said I had an "logical partitions overlap" problem in sda7. So I put in gparted and deleted the proposed linux partitions (keeping the W7 and Data partition). But when I put in Arch again, cfdisk now just says And in gparted from my Ubuntu LiveCD, my entire sda drive is labelled "unallocated". I hope this doesn't mean I lost all my data

Also, it also says the DiskLabelType is unrecognised

Running gparted from the Terminal gives Can't have overlapping partitions

Your mbr appears to be corrupted. This can happen for a variety of reasons especially when doing successive partitioning operations using both windows and linux partitioning utilities. Windows and linux handle partitions a little differently which can cause problems. If your data is still accessible from within windows, I would backup to external media and image your Win7 install to external media as well. Macrium has a free windows imaging application called "Reflect" that works well with Vista so it should probably work on Win7:

http://www.macrium.com/reflectfree.asp

You can try some mbr repair tools that come on some linux livecds like Parted Magic. I don't recall the name of the application but I know they include one on that livecd. I wouldn't attempt that without a backup as the risk of data loss is great.

Assuming you can get a good data backup to an external hard drive, your probably best off deleting all the partitions on your hard drive and starting over. Partition your hard drive with one primary windows partition first and install Win7 on it. Create your data partition after that and then do your linux installation on the remaining unallocated space.

I've repaired it, partially, with testdisk SystemRescueCD. And I can access my W7 and Data partition. But when I try to install Arch cfdisk still says
oddly, when I tried to install Ubuntu in it, it's fine.