Also running STO on a MacBook Pro via bootcamp.
Great advice in this thread so far, hope my additions are helpful.
Regarding a copy of Windows;
As darramouss1 mentioned you may be able to reuse the copy of Windows from your old machine. But depending on the version of Windows and the PC manufacturer it may not be possible reuse your licence key.
jim940's OEM suggestion is an option to save money, if you have no plans on moving the copy to another machine you should be fine.
Another option you may wish to consider is using the
Windows 8 Release Preview (Wikipedia Link) which is currently free to download from the Microsoft website. Keep in mind though that this is beta/preview software and it may not have the stability* of a final release.
Windows 8 is due for release in late October, and its been suggested its pricing will be cheaper than Windows 7 was at release. Regardless, it may be a good idea to wait 'til nearer that time as retailers will likely reduce their prices on Windows 7.
Partition Size
I'll second darramouss1 in making sure you create a healthy sized partition to run Windows on, as its not possible/very risky to change its size later. I'd personally say 50GB minimum (20gb Windows, 10gb STO, 20gb free space for updates/swap file/etc.)
Security Software
My personal favourites and recommendations are; Microsoft Security Essentials for free, or Kaspersky Internet Security as a paid solution. Kaspersky also make security software for Mac OS X, and their Kaspersky ONE product allows you to get both Windows and OS X versions for a good price.
* Stability is a relative term.