I have an oem desktop pc which uses an Intel Dual Core processor and SATA drives.
Just recently it has had problems starting up first thing. It initially won't even get to the BIOS flash screen but the longer I leave it before resetting, the further it gets into start up until after being on for about five minutes, it will get into Windows XP and evrything is fine for the rest of the day, even if I switch off and on again.
I backup data files to an external drive and also have cloned my internal drive to an external SATA drive connected to a back-plate (the PC sees it as a second SATA drive). Yesterday afternoon, I disconnected the internal drive and made the "external" one the main drive and all seems well but this morning it was back to the same problem so I doubt that the hard drive is at fault. I've also given the interior of the pc a good blowout to remove dust from processor heatsink and psu but problem still persists.
Scratching my head as to what to do next.
Anyone any ideas please?

Could be a number of things :

The most likely thing I'd check first is the RAM . Sounds as if your PC might be subject to a bad sector on your RAM module . If you've multiple RAM modules try booting the PC firstly with one module in , see if the PC boots , then try the second . A decent RAM checking freeware tool is Memtest - basically it tests all sectors of the module and will enable you to see if all sectors are ok . If you have speakers attached you'll be able to hear a " beep " error sound with faulty RAM , faulty RAM should still allow boot to POST though . CMOS is responsible for POST screen boot , powered by a small 3v lithium battery - that could also be an option to check . Do you get any discrepancy in time / date settings when the PC is actually working ??

Try disconnecting and reconnecting all power and data leads from the sata drives and rebooting - from the motherboard and not just the drives themselves .

As for the PSU , open the casing and earth any static by touching the open metal of the casing , power up the pc and check that the fan is operational , same for the CPU heatsink and fan .

How long was the case dusty / dirty before you cleaned it ? You might have done damage to the CPU - being the possible most expensive component I'd check everything else first .

Here's a link to a more extensive thread that you might find useful :

PC Randomly Won't Boot

Thanks for your reply and sorry for the delay in responding but PC went haywire yesterday and either wouldn't start or froze very soon after starting Windows. I found a number of errors in the event log and have sorted those, have replaced the motherboard battery and flexed various connectors. PC seems to start OK now (not tempting fate I hope) but has either locked up or restarted at random a number of times. Have now removed one or the two RAM modules and will see how things go.

Problems still persisted so I replaced both memory modules which unfortunately did not solve the problem however, I yesterday, replaced the power supply which appears to have cured all my pc ills!
Thanks for your help.