My pc has been working fine this morning. I went out at 7 came back at 2pm. Pc boots up but will not let me open any files it wont run any programs just stays on desktop, I have made no changes at all. When i try to open internet after 5 mins it goes to black screen and a box comes up saying security failure.
The only programme it will let me run is the anti virus microsoft security esentials and when i run scan it says nothing found.

It wont even let me copy all my files to external drive.

I'm no help
But you have my sympathy
An 'iffy' computer can be very stressful, especially if it contains a lot of personal files such as family photos

I have been struggling with some laptop problems for a couple of weeks. Not sorted yet but at least I have all my files off my laptop and onto hard disk (two disks just to be extra sure).

Snod

During the boot sequence, i.e. after the BIOS POST screen but before the Windows splash screen, press [F8] to bring up Windows Boot Manager:



On this screen, choose Safe Mode with Networking.

Once the machine has booted to desktop, open your browser of choice and navigate to http://housecall.trendmicro.com/uk/index.html or http://www.pandasecurity.com/homeusers/solutions/activescan/ to perform an online virus scan or your computer.

Whilst you are there also download and install AVG Free antivirus and the trial version of MalwareBytes anti-malware.

It wont let me go on internet in safe mode.

Then do not worry about the internet - just use SAFE mode to transfer all your personal files to an external hard disk. Getting your personal stuff safe is the first priority in my opinion. There will be other ways to get internet from SAFE mode

Snod

Safe Mode with Networking enables Internet and network access.

Go into Safe Mode with Networking, then once loaded, go to Start -> Run then type "MSConfig" (Without quotes, of course)

Then go to the "startup" tab

Then go through the list and uncheck pretty much everything as a starting point. Any specific software for your printer/other peripherals may not function until you reenable their checkbox.

Once everything is unchecked, restart the computer into normal mode (Simply restart and leave it to boot on it's own accord) then test out to see whether the problem is rectified.

If it's all fine after that, simply go back to MSConfig and begin reenabling the required programs then restarting the computer.

If you like, post a screenshot of everything on the list and I'll have a peek and see if anything jumps out at me.

Good luck!
Tom

Don't worry about the net in "SAfe Mode", it's to get things sorted and then you eventually get back to normal mode and carry on from there. Use "Safe" as a diagnostic and fixer not as an operating scenario. I've got my own probs but that's another post, good luck.

You need Safe Mode with Networking. The picture is taken off the interwebs, and is perhaps misleading with "Safe Mode" being the option highlighted, so I should have made that bit bold in my original reply.

Also you may need to use a wired connection (Ethernet cable) to the router/modem to get on the internet.

The only reason to do this is to rule out an infection. As just copying the files to an external drive from safe mode has the risk of spreading any nasties to another machine.

Try system restore from a recovery disk.

You dont say what operating system you are using.

I turned system restore off months ago (doh) its vista

Get another hard drive and install it as the master,
reinstall windows on that drive and then connect your own hard drive as the slave and see if it will open the files.
If so copy your files to your new hard drive and your sorted.
I think the problem is your o/s has become corrupted.