Your PC can only be as fast as its slowest component. The problem with your PC is likely involving the hard drive. Its possible that it is fragmented, or you have something running on your system that is requiring costant use of the hard drive.
Because you've mentioned that you are having to restart the computer, you may have problems with software installed and/or malware.
Of course, the easiest and quickest way to resolve this issue is to perform a re-imaging of the PC to restore the PC back to its default configuration if your PC came with a rescue disk/partition. However, that process will wipe the system clean so if you have data that's important for you, consider that before moving forward.
Turn off automatic restart so that you see any error message: http://pcsupport.about.com/od/windows7/ht/automatic-restart-windows-7.htm
If there is no error message then possibly the system has a heat or connection problem. Check the Event Viewer logs for software problems.
REmove any programs, toolbars that you don't use. If you bought the computer preloaded with W7 there likely is a mass of OEM software that bogs your system. There are automatic cleanup tools for some of the worst OEM installations, but it is something you can do yourself. Remove time-limited trials, automatic software update checking, OEM versions of public softwares... like, say, their own-brand AV service.
You can do one thing as to make your PC Fast:
Please close all the programs that are running in background and then
• Go to Run ► Type ► Temp ► Enter ► Delete All the files.
• Go to Run ► Type ► %Temp% ► Enter ► Delete All the files.
• Go to Run ► Type ► recent ► Enter ► Delete All the files.
• and also be sure that you delete all the cookies and toolbars that were downloaded.
• Remove unwanted application from "Control Panel ► Add or Remove ► ..."
and then all after you PC will somewhat fast then before.
As to delete all this files on one click there is one app which is the best app: "Ccleaner" Run it...
• you can also check the registry error and make fix it too using this application.
• This application is from Priform.
• google it out with: "Ccleaner"