You probably have too many autoloading programs. If a brand-new PC boots slowly, you can bet that the vendor weighted it down with too many bundled programs that load automatically with every boot. If a once-fast PC has developed that problem over time, you've probably added too many of these programs, yourself--perhaps without realizing it.
Which doesn't mean you have to remove the programs; you just have to turn off their autoloaders. I use Photoshop nearly every week, but I see no need for a Photoshop icon in the system tray, or a little piece of Photoshop always in memory.
Before you do anything else, create a system restore point: In XP, select Start, All Programs, Accessories, System Tools, then System Restore. Select Create a restore point, click Next and follow the wizard. In Vista, click Start, type sysdm.cpl and press ENTER. Click the System Protection tab, then the Create button.
To see all of your autoloaders, select Start, then Run, type msconfig, and press ENTER (you don't have to click Run in Vista). Click the Startup tab. You can uncheck any of the listed programs to keep them from autoloading.
Which ones should you uncheck? Not your antivirus, firewall, or security suite; you need those running at all times. Other than that, you have to use your own judgment and experiments to determine what you want running at all times and what you want to launch when you need it.