Which partitions do I actually need?

Just had a thought- could the discs not loading be something to do with them being 64bit? Would 32bit be better?

The bit-version of the installers would not affect their ability to boot. Are these known-good CD's which have booted previously OK? If not confirm the iSO and burn another using ImgBurn at 4x speed.

I would use 32 bit with 3gb RAM anyway.

Yeah, the CDs are fine, I use them for a bit of stuff and never let me down, when booting into mac installer it worked fine on the same disc. I assumed because the existing installation was 64bit, the hardware was designed for 64bit and would run it better than 32bit. I'll do new discs and check the speed. How does the speed make a difference?

I don't see any difference between 32 and 64 except that 64 can lag on less than 4gb RAM. You also need 64 bit to use more than 4gb RAM.
32 bit vs 64 bit Comparison - Windows 7 Forums
64-bit More than just the RAM bit-tech.net

Ok thanks for that. Is it right that windows comes with a boot loader? Or do I have to get a third party one?

What type of boot loader are you referring?

If you mean creation of the Dual Boot Menu, these are the best methods:
Dual boot Ubuntu-Win7
Dual Boot Installation with Windows 7 and XP

Trying to boot Ubuntu 32bit from a usb stick doesn't work. it just starts booting windows 7

Have you tried booting the stick using the BIOS one-time Boot Menu key? Does it show up there to select?

If not how was it written to stick. Try Universal USB Installer

I'm not sure. How I do it is, as it boots, it hit esc, the boot device and select the stick. Not sure it that's what you were describing

So it shows up to select on the one-time BIOS boot menu key.

Then there is a problem with the stick or how it's written. Try writing another.