Updating OSX

Bought Snow Leopard from Apple - installed off the disc. Then ran software update and added system update then went to App store to install Mountain Lion. This or any other App for that matter will not respond when I click 'buy app'. Tried this twice with same result.

Am on:

Hardware Overview:

Model Name: MacBook Pro

Model Identifier: MacBookPro5,1

Processor Name: Intel Core 2 Duo

Processor Speed: 2.4 GHz

Number Of Processors: 1

Total Number Of Cores: 2

L2 Cache: 3 MB

Memory: 2 GB

Bus Speed: 1.07 GHz

Boot ROM Version: MBP51.007E.B06

SMC Version (system): 1.33f8

Any ideas before I throw in the bin?

Thanks

P.