Windows 7 - PC freezes every day after about 15 minutes

My work PC freezes after about 15 minutes from starting. I can't do anything at that point and I need to restart it by pressing power button. After second start it runs normally for the rest of the day. Next day when I switch it on it happens again and it's been like that for the last week. It froze on IE, Firefox and Outlook. I checked event viewer but don't really know what to look for as only errors that I've found were that of unexpected machine restart. I've checked installed programs and there's nothing there recently. Any ideas?

it still happens. yesterday it worked for an hour and then it froze but today was about 10 minutes.

Blue Screen of Death (BSOD) Posting Instructions

Sounds like the cold boot issue, usually caused by the RAM.

What is you RAM card configuration?
Which Dell do you have?

Run memtest86+ you are looking for bad RAM cards, bad motherboard slots, so you need to follow the testing procedure in the tutorial.
Sometimes errors from these tests can be corrected by adjusting the settings.
RAM - Test with Memtest86+

Can you download CPUZ and post a snip of the CPU, Mainboard, Memory, and SPD tabs.

For posting the information , enter 'Snipping tool' in the Start button > Search box, hit enter. Select the area you want to post and save to a convenient place.
How to Post a Screenshot in Seven Forums


Have you been checking your temperatures?

Use Speed Fan or Real Temp , run one of these and check the CPU temps when at idle, under load and/or testing.
Let us know the minimum and maximum temps you get.
Go to your BIOS and tell us what the CPU, RAM and North Bridge (NB) voltages are.