Overheat due to faulty / blocked fans
Household power surges fridge etc switching on/off
faulty memory stick try memtest http://www.memtest86.com/
virus or spyware scan for both especially if shutting down with in a few minutes of switch on or internet use
driver conflicts especially grahics cards get the latest drivers for your equipment
a) Clear out all temporary files and folders -- use CCleaner http://www.piriform.com/ccleaner/download
b) Scan for malware spyware and viruses --Free Anti Spyware :- Malwarebytes http://www.malwarebytes.org/ Superantispyware http://www.superantispyware.com/download.html
Free Antivirus software MicroSoft Security Essentials http://www.microsoft.com/security/products/mse.aspx Avast http://www.avast.com/en-gb/free-antivirus-download
c) Clean the registry -- Use the tool in Crap Cleaner its very safe and also allows you to back up the registry first.
d) Pagefile (Virtual Memory) -- Right click MY Computer - select properties - Advanced tab - Performance - advanced tab - Virtual memory click change, you can put the page file on a different drive (if you have one), click custom size and set Initial size to one and a half times the amount of memory you have fitted i.e. 512MB memory = set to 768MB, set maximum to double your memory amount i.e. 512MB memory = 1024MB click ok. If your hard drive is full and there is not enough room for the pagefile this can slow down, freeze or even cause the PC to crash (restart).
e) Cut down on the programs that load at start up -- Start - Run type msconfig - start up tab- untick everything except for firewall, antivirus and antispyware
and the services that run in the background. http://www.blackviper.com/
a) Hard drives /
i) IDE Channels: (Not required if you have SATA drives) Check the transfer rate, you need to have the transfer mode set to DMA not PIO. Right click My Computer - Properties - Hardware - Device Manager - Expand (click the + ) IDE ATA/ATAPI controllers right click Primary Channel - Advanced Settings Tab - If transfer Mode is PIO then follow the instructions at http://forum.digital-digest.com/showthread.php?t=61905 to change.
ii) Check for errors and defrag your hard drives -- My Computer - select drive - properties - tool tab - Error checking / Defragmentation.
ii) If you are using Windows XP, Vista or Windows 7 it's a good idea to convert your system drive to the NTFS file system if you have not already. In addition to providing numerous security and data recovery improvements over FAT32 (the file system of choice for Windows 9x/ME and XP Home) it can also speed up your system slightly.
In fact, the only real reason for sticking with the FAT32 file system for any of your data is if you have more than one operating system on your PC and the other OS's can only see FAT32 partitions (as would be the case with Windows 98, for example, which is incapable of reading NTFS data).
To convert your drives to NTFS: Right click on 'my computer' and select 'manage' From the computer management window, expand storage and select 'disk management.' Using the 'file system' column of the upper pane of this window, you can easily check what file system each of your logical drives is using. Make a note of this information. Now open a command prompt window by going to 'start\run' and typing 'cmd' To convert a disk to NTFS, type 'convert (drive letter): /fs:ntfs' So for example, if you were going to convert your C: drive, you would type 'Convert c: /fs:ntfs' at the prompt.
b) Drivers Obtain the newest drivers for your hardware This may seem a bit obvious, but keeping your system's drivers up to date can give both your performance and stability a boost. Video card manufacturers release updates especially often, and these can often give "significant boosts" to gaming performance as video card in question is "optimized."
Don't neglect the other components of your system either. Your motherboard manufacturer may have released newer versions of its Input/output drivers for your board, and sound cards and other peripherals can also benefit from newer software.
c) Memory Add more physical memory, this of course means opening the "box" and fitting a memory module, make sure you buy one that is suitable for your PC. Crucial .com http://www.crucial.com/uk/ will guide you through the process of selecting the correct memory. http://support.gateway.com/s/Manuals/Desktops/8509270.pdf for a guide to fitting memory.
ignore 1b has you have Virgin media security but do a scan worth downloading and scanning with Malwarebytes.
I suspect you may be having a RAM problem.