MBR Partitioning table: Maximum of 4 partitions (3 primary + 1 Logical drive). If you make an extended partition out of shrunken space, and move the contents of the HP partition into a Logical Drive on the Extended Partition, it should be fine.
Of course the solution to this is GPT, 18EB Partitions and 192 of them ftw...
anyway, Wubi is the Windows Installer for Ubuntu (Windows-based Ubuntu Installer). You only use Wubi if you install or uninstall Ubuntu from inside Windows. If you boot from the CD and do it, then it would be irrelevant.
You can install Ubuntu through Wubi onto the same Partition as Windows, and it will save it as an application but add it to the Boot.ini/BCD file. This isn't recommended but it would work fine if you have run out of partitions to put stuff on.
also, i recommend you use FAT32 instead of EXT4 if you're going to do any formatting for it, cross-compatible file system see.