When I do a Google search, the usual page of results appears, but when I click on any of the results, instead of openning the page on which I clicked, a strange page appears which seems to be a commercial search or list of possible links. Is this a spyware invasion? How do I stop this from happening?
Thanks

Do a quick search on your hard drive for a file HOSTS
A normal file will not have any website addresses listed. Many spyware programs will add their own entries to this file.

1. Download and run Firefox to protect your from future spyware attacks and pop ups which are coming in through Internet Explorer IE! (Trojan downloaders, win32 ).
Security News from the net: spyware fighter essentials (http://securitynewsfromthenet.blogspot.com/2007/05/spyware-fighter-essentials.html)

2. Run the vundo and combo fix Security News from the net: vundofix and combo fix (http://securitynewsfromthenet.blogspot.com/2007/05/vundofix-and-combo-fix.html)

3. Run Malwarebytes Anti-Malware
Security News from the net: Malwarebytes Anti-Malware (http://securitynewsfromthenet.blogspot.com/2008/03/malwarebytes-anti-malware-105.html)

4. Run the anti spyware remove programs spybot Security News from the net: Spybot search and destroy spyware and popups (http://securitynewsfromthenet.blogspot.com/2007/03/spybot-search-and-destroy-spyware-and.html) and superantispyware Security News from the net: SUPERAntiSpyware Home Edition (free version) (http://securitynewsfromthenet.blogspot.com/2007/04/superantispyware-home-edition-free.html) to get rid of the nasties

5. Run a complete scan with free curing utility Dr.Web CureIt!
Dr.Web CureIt! – download free anti-virus! Cure viruses, Best free anti-virus scanner! (http://www.freedrweb.com/)

Malware & Spyware Removal Guides (http://www.bleepingcomputer.com/malware-removal/)