Ubuntu installation - partition problems

Hi,

I'm trying to install Ubuntu alongside Windows 7 on an HP and I've run into some problems with the partition. I've used the standard Windows tool to shrink the C: disk ~100GB and left this space as unallocated. The Ubuntu installation however won't let me install on this (there's no "install alongside windows" option - I've read I shouldn't use that anyway? - and the unallocated space in the "something else" option is listed as unusable). So I've read up on MBR and have changed one of my 4 partitions (tools) from primary to logical/extended.

Now the whole disk is split into:
- system for windows, 200 MB, primary
- windows disk space, 800 GB, primary
- unallocated space, 100 GB
- recovery for windows, 18 GB, primary
- extended containing hp tools as logical, 100 MB
- unallocated space, 1 MB

I thougt maybe moving the unallocated space into the extended partition would help, but I'm not sure and I don't know how to do it. Can anyone please help me? This has already taken days and is starting to be really frustrating ...