I have an Apple imac and am using a 120GB portable hard drive as my time machine dish. I would also like to use the disk as a portable hard drive but is not recognised by other PC's. Is there a solution anyone can give me or will the drive not work. Thanks in advance.

I created two partitions like this guy did, one for Time Machine and one for Windows:
http://balloflightning.com/2009/02/dual-partition-a-usb-drive-for-time-machine-and-windows/

Alternatively, you could try something like Macdrive for your PC to get write access to the external drive. Not sure I'd trust my backups if they were sharing a partition being accessed from PCs running Macdrive though.