which operating system ?
Normally your computer will turn on a screen saver or enter sleep mode after a certain period of idle time, to protect your monitor from suffering a "burn in" of an image that appears on the screen for extended periods of time. This also helps to save electricity during periods of inactivity. Vista owners report via Internet tech help forums a seemingly rampant "bug" that causes the machine to freeze after being left idle for long periods. No single factor seems to cause this, and instead requires troubleshooting and trial and error to find a fix that works. Screen Saver Failure
When your computer freezes after a long idle period, it could be due to a malfunctioning screen saver. Turn your screen saver off, by clicking "Start" and then "Control Panel." Under "Appearance and Personalization," click "Personalization" and select "Screen Saver." Select "None" under the screensaver drop-down list and hit OK. Pay attention next time your computer goes idle for reappearance of the problem. b> Power Saving Options
b> Windows Vista provides users a way to select how their computers behave after sitting idle for a certain amount of time. In many cases, the hard drive and monitor are instructed to enter sleep mode or even shut down after they've been idle for a specified time frame. To check or alter your settings, click the "Start" button and then "Control Panel." Under the System and Maintenance icon, click "Power Options." Under this tab you can change the amount of time before your system goes to sleep, and the actions it takes after reaching the idle time. Malware
b> When mysterious things begin happening to your computer for no apparent reason, there's a good chance a malware program is behind it. Malware are nasty, hidden programs that download themselves onto your computer to cause havoc and even steal your personal information. These invaders monopolize your CPU, clog your RAM, and generally make using your computer unpleasantly slow and frustrating. Their presence can affect everything from program operation to operating system functionality. Always update and run malware protection software regularly to keep these pests out of your computer. b> Update Windows and BIOS
b> Your computer's operating system and BIOS files are responsible for proper operation and functionality. Over time, these files can become outdated, requiring patches, fixes and other updates to correct compatibility and security problems in your computer. Microsoft provides regular updates for its Vista operating system to correct reported problems. The BIOS in your computer is programmed into the motherboard, so the board's manufacturer may have an update available for your version. In many cases, the brand that sells the computer may offer a link to the BIOS update, so check with Microsoft for any updates available. b> Too Many Processes
b> Your computer will register as "idle" if you haven't used it, even if you have programs open on the screen. After a certain amount of time, the system will send the data you were working on to the RAM as the system settles into sleep mode. When trying to wake the system after this idle period, it may have a hard time resuming the programs that are open, causing a freeze. Close down any program you are not actively working in to save the CPU from unnecessary work, and lessen the load placed on the RAM when it's left idle. right mouse click in the bottom toolbar select task manager applications if you see something running that should not be running select end now you might even uninstall that specific program if its no longer needed also in the processes tab see whats ticking over in the background might be a virus or malware. Download and install at least one of these all free depending on your operating system http://download.cnet.com/avg-antivirus-free-2013/3000-2239_4-10320142.html
avg free 2013 http://www.majorgeeks.com/Microsoft_Security_Essentials_for_Windows_d6242.html
microsoft security essentials your operating system must be registered Win7/Vista/XP 64 bit http://majorgeeks.com/downloadget.php?id=4281&file=1&evp=dbb3b0aebe6a6a4ff18089a6489a5e62
anti malware free edition Win XP/2003/Vista/Windows7 http://windows.microsoft.com/en-US/windows/products/security-essentials/product-information
malware protection http://100-downloads.com/download.php?p=615
windows xp microsoft security essentials http://100-downloads.com/download.php?p=614
windows 7 microsoft security essentials Hope this helps.