How recent is the install? Is there a lot of drive activity? Does Task Manager indicate what is using all the CPU?
Will it Start and run correctly in Safe Mode. Can you check out how it runs if you login as any other user in Normal Mode?
To get further information about any errors look in your Event viewer.
Look in the System or Application folder. You can get to the Event Viewer via right click My Computer icon and select Manage.
Any errors logged in the Event Viewer can be expanded by double clicking on the error line.
Take any event error I.D. number and search for it on this site.
If the problem occurred after installing a device driver or application, try removing the driver or program.
Any errors in Device Manager?
To check conflicting software, most of these apply equally to Vista.
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Removing adware & spyware
faq608-4650
Will check your computer for spyware and adware.
See if you have any services that are flagging as "Starting" but not actually running.
Some general things to try.
See if System Restore will get you back to a restore point before your problem with Windows.
Try running ChkDsk to check your drive for errors. Right-click your Drive icon/ Properties/ Tools/ Error Checking. Select both boxes.
Run the System File Checker program from an elevated (Administrator) command prompt by typing.....Sfc /Scannow in it.
Windows Help and How-to