Well you can get a Windows install dvd (most versions will work) and format the partition which has the trail version of Windows 7 on it.
You can see how many partitioned disk spaces you have (i.e. c:, d:, f:, g: etc) by going into your BIOS(reset computer, wait for beep (if no beep then just press the button from the start) press del or f2 or whatever to get to BIOS) then look for "BOOT OPTIONS", and see which drive is the primary boot drive, and which hard drive (partition) is set as the primary HDD, and set it to the other partition (or atleast the one with your previous OS on it).
If you only have one HDD with one partition on it then I'm afraid your old OS was probably completely wiped off when the trial version was installed.
Fear not the BIOS!!!!