Windows startup problem

[ayachan771]

Hi, When I start up my computer, a small window comes up titled Windows Activation. It then goes on to read " An unauthorized change was made to windows. Windows has discovered a change that will result in limited windows functionality. Use the link below to find out how to fix windows. "
It then gives me the option to Learn more online or Close. Closing takes me back to my computer login. And the whole problem just links around again.
When I click on Learn more Online, the link, it goes to a web page which stated that "Windows validation was interrupted. The validation service is unable to determine whether windows is genuine." It then asks me to pay to 'renew' my windows version.
I have run spy bot Search and Destroy a number of times, as I am guessing it is a virus?? but to no avail.
The only way I can then use my computer, is to fully shut down and reboot...which usually lets me log on sucessfully.
I would appreciate any herlp with this problem. Thanks.