Hi
well, the out of the box archiving functionality was introduced in 6.x I think. Bad luck for you 
If you want the archived data to be available in the system, I'd manually create archive Forms (Save Form as) and some filters pushing the data to the archive forms. Condition could be 'Status' = "Closed" and 'Create Date' < $TIMESTAMP$ -x and/or some Flag. After you pushed the data, you'd have to delete the requests from the original table of course.
If you don't need the data in the system you could search for it in User Tool and Export it to *.arx files. Afterwards delte it.
Cheers
Marcus