Medieval 2 campaign problem

There's something seriously wrong with the campaign on my medieval 2 game and this is the first time playing Med 2.

I bought it today from a second hand store (might explain why), got it installed and everything was fine until as soon as I started the campaign, it came up with "medieval 2 total war shell" and some control instructions, then it showed the loading screen and as soon as it finished with the loading bar it showed the campaign map for a split second and then went back to the loading screen and repeated this process over and over again.

My computer is a windows 7 64-bit operating system on a Dell Inspiron 1545 with a Pentium(R) Dual-Core CPU T4500 @ 2.30 GHz processor

Please let me know if I can fix this whether it means it's a problem in the files, the computer, or if I should just return the game.

Thank you for any help