Some people want a magical method. Aside from eliminating unwanted and unneeded programs at startup, you could do a fresh install of Windows. Driver conflicts, overheating hardware, and too many programs running will sometimes cause a PC to slow down.
Make the switch over to a SSD as a boot drive.
Add more RAM
Upgrade the CPU
Keep in mind that when you throw faster hardware at the problem, it only raises the threshold for how many programs you can have running. This might be worth considering if you already have cleaned up the Operating System.
I'm sure someone will say, use Linux, but again, this only takes care of the software side of things.