There are 2 steps to this; Cleaning your startup items and Cleaning your services:

Pre-messing with stuff:
- I advise you to set a system restore point before doing any of this so that if you mess things up you only need restore to try again
- If not i advise you at least make a registry backup
Both these tasks can be googled for information on how to do them...

Step 1, Cleaning your startup Items:
- Click start and type msconfig.exe (in the search bar/run) And press enter
- In msconfig go to the startup tab
- Look through these, some/most will be unneccesary...
 > Google the names of the "Startup Item"s and work out if they're necessary for windows to start.
 > Usually things like quickstarters (e.g. realplayer/open office) will run and they're unnecessary
 > Things like your firewall, windows executables (in C:/windows/...) are usually necessary yet again if you're unsure then google

- Okay decide what to disable and just take the tick out of the checkbox, i advise unchecking whatever isn't neccessary as it will speed up starting up

(Just to show you what windows needs i have 6 startup items that are necessary most are my laptop's drivers)

- Once you've unticked stuff press "Ok"

This will speed up your startup conciderably [normally]


Step 2, Cleaning your services:
- Click Start
- Type services.msc in the search bar/run and press enter
- These are your services, click the columb header "startup type" to order these by the startup type (you'll have all the automatic startup items at the top)

- Go through these services (that have an automatic startup type) and research them again(google)
- Work out if they're unneccesary
 >services are more risky than startup items so dont disable them if you are unsure
- If unneccessary:
 > right click the service
 > click properties
 > change the startup type to manual (in the dropdown box)
 > click ok
- restart your pc when done disabling all the unneccesary ones
 >there wont be many here

Your PC's startup time should be alot quicker