My laptop wont let me log on keeps logging on then logging off straight away

Your system is infected by trojan,

Scan your System with a good Anti-Virus like Kaspersky Internet security 2010 or A-Squared Anti-malware with lastest updates installed.Both can detect trojan as well as malware,spyware etc.

This will solve your problem.

Hi. True, your computer is probably infected with malware/viruses or your Operating System is corrupt. You can try the following steps: 1. Unplug all external devices. Remove the battery and ac adapter. Press and hold the power button for 10 seconds. This will drain excess power. Plug back in the ac adapter and ethernet (for your internet connection). 2. Upon turning on the computer, press F8 once every second. You will get the Advanced boot menu, select "Safe Mode with Networking". (Internet connection works in this mode) 3. Select your Operating System (Windows version) then login to your user name. 4. If you're successful, download and run a scan (Quick and Full) using Malwarebytes. It is a free software from www.malwarebytes.org It is safe to download this file. No need to purchase the paid version. I tried this, it works. 5. If you are unable to login in any mode, try a repair. If it does not work, slave the hard drive to make a back up before you Reinstall Windows. You can also consult a local repair shop for data recovery (but this is fee based).
I hope this helps.

Hi,your laptop is probably infected with a virus,so there is one of two ways we could these,if you can slave the hard disk to a system with a good antivrus like norton or mcafee and up to date definitions installed and do a manual scan to rid the harddrive of infections or you can clean up your entire system and start afresh.NB. after the scan you may need to repair your Operating system.