Connect To PCs Via USB?

I have reached maximum capacity on my laptop's 80 Gig HDD and I am wondering if there is any way I can connect the laptop to an older PC I have lying around to sort of use the older PC as a kind of external storage device... Please don't laugh at me, I'm learning as fast as I can!

 

http://www.windowsnetworking.com/j_helmig/usbmain.htm

Although my choice woiuld be to use a CAT-5 crossover cable instead.
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I don't know if this will be of any help or not, but here goes: The laptop is a Toshiba Satellite P25-S509, and the desktop is an HP Vectria(?). I do have a 9 pin serial port that I am currently using for a second monitor, as the LCD on the laptop was smashed via a woman scorned. There is also a 25 pin serial that's not in use, but I do have 4 USB 2.0 ports free! As for the gigantic, aging HP, there are two 9 pin, and one 25 pin serials. My reason for wanting to use the USB method is that I already have the USB cable with the connector between the two cords, and I'm not financially able to justify the trip to Radio Shack for a two way serial. Both machines, however, do have Ethernet cards, if that would make any difference in the operation.
I am desperate to accomplish this mission as frugally(?) as possible, as the Ol' Lady would have my a** if she found out I was 'blowing' money on this project, and I'm affraid the HDD in the laptop may be on it's last leg. I have tried to find a replacement HDD, but Toshiba only deals replacement parts through a wholesaler who doesn't even list the drive that I need!?