Its all happening because of virus, install any good antivirus ,update it and run a full system scan timely. Do not install lots of applications or any documents to desktop of your computer because it effects computer performance so moves the files to my documents folder or bettar still a dedicated hard disk. Go to Run by pressing Win+R shortcut key then type msconfig and press enter. Startup window appears on the desktop screen. In that window, at startup tab, uncheck all items that are not needed at the startup. This will decrease the bootup time. Yes, your computer needs a software called CCleaner which will scan, clean and fix all the registery, applications, internet cokkies, internet caches of your computer. After that Reboot your PC then see it works or not if the problem still persists than you have to install new version of windows or another operating system for your computer. If you install new version of windows or another OS only multimedia files such as JPG, Audio, Video.., etc. will stored in your hard disk but extension files and installed applications are permanently deleted from your PC.