After asking here I deleted my entire ubuntu install and reinstalled xubuntu as per instructions here. I've partitioned 20gb root, 478gb home, 2gb swap, on sdb (a non-booting windowsXP is on sda). I installed xubuntu from liveCD onto sdb, which put grub on sda which is recommended for this exact situation by this post from this thread.
My PC now doesn't boot - no grub menu, just a flashing underscore. I redid the install - deleting partitions and re-doing them, just to make sure it was putting grub on sda. I can boot to LiveCD and use as normal. Any idea what I should do?

Thanks in advance.