Hi Krlea,
Time of year doesn't matter too much, the weather is so unpredictable. The best times I'd say would be summmer (July/August), or in the run-up to Christmas when everything is lit up with pretty lights and its very beautiful. You can also get some excellent xmas shopping done!!
I'd take a few days to see the sights. The trouble with London is that its very spread out and the longer you have the better. IMO the best things to see/do are a boat trip down the River Thames
The London Eye
and of course lots of shopping.
About 5 days would be enough I reckon. I mean, there are lots of things to do if you stay longer, but it really depends on your time and budget.
Don't rent a car. Like Turnbull says, the public transport is excellent (except when like me you have to take it into work every day!), and there's nowhere to park. You also have to pay to drive into the centre of London now.
Anyway, just let me know if you have any questions I'd be happy to help out.