A computer's startup is largely affected by the number of programs that launch on startup, and most of these programs aren't needed. You can easily change which programs will launch on startup, using a utility called msconfig.
Go ahead and press Start, Run. Type in msconfig and press enter.
Then, select the tab Startup. Here you can see all the programs that launch at startup.
From here, you can simply uncheck your unwanted programs. Things like Yahoo Messenger don't really need to be there, whereas you might want to keep Skype open to receive calls.
If you are unsure of something, expand the 'command' tab and you'll see where the file exists. And don't uncheck any system files, please.