1) Hit 'start' and type in "msconfig", go to the 'startup' tab and disable anything that is not needed when you boot up (for example you do not need every adobe updater on earth to start with your PC, nor do you need skype to start with it, but you may want to keep things like antivirus, mouse/keyboard software, and other commonly used things).
2) download CCleaner and run both the cleaner and the registry checker. Rerun the registry checker until there are no errors, or 1-2 errors that it cannot resolve.
3) use a lightweight antivirus such as Microsoft Security Essentials
4) be sure to install all of the drivers for your motherboard, GPU, and other devices. Even without the proper drivers it should run fairly quick, but with the proper drivers things will run much smoother.
If none of those work then it may be a hardware issue. use the windows utilites to check your RAM and HDD for issues (windows memory diagnostic, and Check Disc). Check your system temperatures as all modern CPUs will slow down if they are running too hot.
What HDD are you using? If your are reusing an older drive then a new HDD or SSD will speed things up considerably.