Are you sure you are not disconnecting from the Internet, at any time ?
If the problem seemed to have started after installing something, as you stated, you should first try uninstalling that software.. Start>Control Panel>Add remove programs>... find the program/tool that you installed, highlight it and click remove..
If that doesn't work, post in the virus-removal forum here to check your computer for viruses, to out the possibility of viruses..
If viruses are not the problem post back here to continue the troubleshooting process...