Thanks for your advice everyone! I Have seen this... anygood? http://www.johnlewis.com/231577589/Product.aspx
As I said above, put DPC latency checker on a usb stick, go into John Lewis and ask them to let you run it on the model you are interested in.
Also, re security, it's a state of mind rather than something you set and forget. As other people have said, make sure your firewall is on, you pc is patched to the latest version and install a good anti-virus (I use avast).
Macs suffer less from viruses at the moment. That's because they aren't targeted by virus writers not because there is a magic bullet.
Good luck with it!