The GPT specification clearly states that if the MBR contains no protective GPT entry (one with a type code of 0xEE), that it's not a GPT disk. Your disk has no type-0xEE MBR partition, so it's technically an MBR disk, not a GPT disk. Furthermore, since you can boot Windows, Windows is clearly using the disk as an MBR disk, so that's what it is from a functional point of view. At best, the GPT data duplicate your existing MBR partitions. At worst (and this is more likely), they provide contradictory partition information that, if some tool were to try to use it, could end up damaging your real partitions.
Your immediate problem is that libparted is getting confused by the valid MBR and leftover GPT data. You've got to clean out the leftover data.
It's currently in MBR format. The problem is that whatever utility you used (probably the Windows installer) did an incomplete job in converting it back, in that it didn't wipe all the GPT data.The whole GUID makes sense now. About a month ago I tried to do a hackintosh and couldn't get it. I decided to just reinstall Windows, so did I forget to format my hard drive back to an MBR?
It's unclear what was unable to access your Windows 7 files. Was this the Ubuntu installer in "live CD" mode? Something else? In a normal Linux installation, it is possible to access Windows files. I suspect you either didn't know how to do this or there was a problem related to the leftover GPT data. In any event, I recommend you put this question off until after you've installed Linux, with one caveat....Also, I noticed that I wasn't able to access my Windows 7 files. When Ubuntu installs, will I be able to?
It's unwise to regularly access any OS boot partition from another OS. In this specific case, Linux doesn't understand the security features that Windows uses on its filesystem, so it's easy to accidentally trash your Windows setup as an ordinary user in Linux. Also, although Linux's NTFS driver seems pretty reliable based on anecdotal reports, I find it hard to believe that it's as reliable as Windows' own NTFS support.
Thus, I recommend that when you repartition, you create a separate FAT or NTFS partition for shared data -- any files you might want to access in both OSs (MP3s, photos, etc.) or files you might want to transfer between OSs (if you download a Windows file in Linux, say). Mount and use this partition in both OSs, but when you're in Linux, do not mount the Windows C: partition. That'll keep the OS installation itself safe from damage.
If you wipe the GPT data, they will trouble you no more, unless you mess around with tools to convert the disk back to GPT form. I'm not sure what you mean by "installing any other files."Finally, if I get Ubuntu installed, will I ever have any more problems with the GUID partition table or installing any other files?