For example, in thw Windows search bar type: Acer restor, or Sony restore depending on the model of your system.
This is to see if there is a pre-installed program which alows you to restore your system back to factory settings.
If not you may have to go into BIOS to do this, if so, for example just Google: How to factory restore my Acer 6935G (Make sure you include the make and model of your system.
Maintaining your computer
1. Left click on C drive> properties/run windows disc cleaner
2. Download Ccleaner from here: http://www.filehippo.com/
3. Install Ccleaner and run the cleaner top left, then run the registry cleaner below it on the left. you don’t need to backup the registry
4. Left click on C drive> properties>Tools> check now/disc scanner or in the case of Vista it’s called something else > tick both boxes > start, Prompt the scanner to scan upon next reboot and REBOOT COMPUTER)
#(Prepare to wait up to an hour)#
5. After Disk scan/Disk check has finished run windows Disc Cleaner again, Ccleaner again and registry cleaner again.
6. Then you must do a DEFRAGMENTAION of C drive which is START>ALL PROGRAMS>ACCESSORIES>TOOLS>DEFRAGMENTATION
Additional options: Remove current anti-virus as well as any adware and spy wear.
Free Microsoft Essentials Anti-virus http://www.microsoft.com/security/pc-security/mse.aspx