I have a 1995 Scott Magnum hybrid bike.

I'm just getting into road cycling, I've done a couple of longish rides approaching 100 miles and I really enjoy riding the bike. The only problem I've found is that at maximal peddling I can only achieve around 32kph. I'm wondering what I can change to get a better gear ratio thing going on. I don't have the money to buy another bike and I quite like the fact this one can handle a bit of mild off-road.

It has:

Wheels 26inch
Crankset Shimano STX HyperDrive-C, 22/32/42 teeth
Rear Cogs 7-speed, 11 - 28 teeth

I'm just starting to get my head around all the terms and parts so I'd appreciate some simple advice that I can research a bit.