download the sysinternals suite from sysinternals.com and run autoruns and click on the logon tab to see what is starting up with Windows.
Items from Adobe or Java quicklaunch should be unchecked so that they do not slow down startup, as well as steal cpu cycles.
Some apps however, such as Photoshop and anti-virus, do need their startup items enabled for the apps to work correctly.
Using the logon tab in autoruns can allow you to check on every startup item and confirm whether it's needed or not. Basically, if it's an app that's already installed like Adobe Acrobat reader, and can be started from the start menu, or like Java, which can be called automatically by the browser when required, does NOT need to be enabled in Autoruns.
You can also run Procexp and see what is actually running.
Everyone here seems to be focusing on hardware. It's been my experience that it's been software, usually several unneeded startup items which slow a PC down and cause it to stutter and lag. Disabling these by unchecking them, then restarting the PC has cured lots of 'slow' and 'freezing' PC issues, as well as speeded up startup and PC performance.
Since you're running Windows 7, I'd also suggest installing and using All CPU Meter and Network meter from addgadgets.com. These will tell you your current CPU speed, how many CPU's are running and memory usage as well as upload and download speeds. Both apps are free.
Good luck.