Internet really slow in OS X but fast in Windows

[oceanscape]

I have Windows 7 installed alongside OS X. When I boot into OS X the internet is painfully slow and keeps cutting out - regardless of whether I use Safari or Firefox.

However, when I boot into Windows 7 the internet is really fast and runs as it should.

Please can anyone help me fix this? I prefer using OS X but with the internet problems it's such a pain.

Thanks!