You have three choices here, two of which are mentioned above.
1) In line converter (works ok, can be slow but not always)
2) Add a USB port (better because you get 2 ports which will allow you to use another USB device in the future along with the scanner and it won't clog up your current parallel port)
3) Exchange the scanner. Many scanners come in both a USB/serial and parallel versions) This choice will be the cheapest, but then you are stuck with a parallel scanner if you get a new PC with USB capabilities.
Then of course there is Chris' choice of throw everything away and get a Mac!
Without knowing any other information, I'd choose #2.
BTW not to confuse you furthur but you can, as Bob has suggested, chain MANY devices from both types of ports, not just 2. If you do use an in line converter it has to be the last device on the chain. Honestly I've never used a converter in a chain, so I don't know how good the results would be.