As for Hardware my 2009 vote has to go for a new Solid State Drive. A good one like Intel X-25M. Made my laptop feel super responsive. Hoping Intel or Samsung releases large capacity drives for my desktop. SSD drives has been the best upgrade I have done in a while.
I've had great success with cleaning up rogue files and registry settings with CCleaner http://www.ccleaner.com/ It's a freeware and it's great!
For a trading PC I don't think formating regularly is worth it if regular maint is done. A trading PC should be like a server that is dedicated for the task. So you should not have problems of installing many different programs or system changes. You run your trading applications and that's all. No gaming, downloading movies, porn or music and you should not have any problems.