You can possibly look at limiting which services run at startup; these are similar to the startup processes but are mostly invisible and take up very little processing space to begin with. It's only for the most fussy of users and not for casual messing; several services are interlinked and stopping one that you don't need doesn't necessarily mean that you won't be stopping a service that you do
If you want to access it simply open up Task Manager (Ctrl+Alt+Del; or right-click taskbar and select 'Start Task Manager'); then head to the Services tab. From here you can't do anything except for see which services are running or stopped, but in the lower right the further 'Services' button calls up a new window allowing you right-click each service individually and choose from a series of options.
In the Properties for any service you can see the service name, a description of what it does, and toggle whether to have it startup:
Automatic (as soon as the machine boots),
Automatic with a delayed start (starts up with the machine, but not the second that Windows loads, so that individual services load mostly separately from one another, theoretically taking the strain off the CPU)
Manual (services will only start when specifically required; for example you turn the Printers services to this setting, and it will only activate when you try to print a document)
Disabled (speaks for itself)
As I say, this is not for the faint of heart nor for mucking about. You can seriously impact your machine's performance by disabling services, or render it inoperable in the worst case scenario. I recommend using a good services guide such as at The Elder Geek Home
You can also use Google to research each one, or in the Properties window for service, click on the 'Dependencies' tab to see which services rely on that particular one, and also which ones it relies on in order to function.
I strongly recommend only researching this method first, though. Research first, act upon it another day if you still feel you can bleed some more performance out of your machine.