First off, do you have to backup any data? Do this first before attempting the Win7 install.
Next, place the Win7 disk in the optical driver & set the BIOS to boot from the CD-rom first.
Follow the Win7 installation procedure, opting to format the C: partition. This will wipe the drive of all data - hence my earlier suggestion to backup if you need to. This will make it free, so you can install Win7 without any lack of space issues.
I've installed Win7 on a 15Gb partition without any errors, so there shouldn't be any problems at all with a 30Gb one. Keep the system partition at 30-40Gb, which is the least you should choose really (Btw, I chose 15Gb as the disk was only 20Gb in total). G
P.S. Step-by-step guide with screenshots at click here if required