Why does Ubuntu 12.10 not detect my Win7 partition?

I'm trying to install Ubuntu 12.10 alongside Windows 7 on my ASUS laptop and I'm new to all of this.

I'm using the Ubuntu 12.10 64-bit ISO burned onto a CD.

The first time I tried to install it, Ubuntu said it did not detect any other OS, which meant I only had 4 options:

• Erase disk and install Ubuntu (I don't want to do this)
• Encrypt new Ubuntu.
• Use LVM.
• Something else.

If I choose the Something else option, it brings me to the partition menu and says that I have 1 disk with free space of 750516Mb (750Gb), but that's not true because I have Windows 7.

I've already read through several questions and people have mentioned that you could only have 4 primary partitions. So I got hold of partition magic and removed my D: and E:, leaving me with primary C: partition and System reserved partition and a lot of unallocated space.

I restarted the laptop several times and booted the CD again and I got exactly the same as I did previously.

How do fix this problem and install Ubuntu alongside Windows 7?

EDIT 1

Here are my results for

sudo fdisk -l; echo; sudo parted -l; echo; mount; echo; df -h

WARNING: GPT (GUID Partition Table) detected on '/dev/sda'! The util fdisk doesn't support GPT. Use GNU Parted.


Disk /dev/sda: 750.2 GB, 750156374016 bytes
255 heads, 63 sectors/track, 91201 cylinders, total 1465149168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x90050371

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048      206847      102400    7  HPFS/NTFS/exFAT
/dev/sda2          206848   146368214    73080683+   7  HPFS/NTFS/exFAT

Warning: /dev/sda contains GPT signatures, indicating that it has a GPT table.
However, it does not have a valid fake msdos partition table, as it should.
Perhaps it was corrupted -- possibly by a program that doesn't understand GPT
partition tables.  Or perhaps you deleted the GPT table, and are now using an
msdos partition table.  Is this a GPT partition table?
Yes/No?  

After selecting YES

Yes/No? yes                                                               
Error: The backup GPT table is corrupt, but the primary appears OK, so that will
be used.
OK/Cancel?  

I had a similar issue and did the following:

• made swap and Ubuntu root filesystem (ext4) partitions with acronis
• left 1GB of free space (unallocated)

After this it saw my ext4 and swap partitions and I was able to install Ubuntu.