First disable your antivirus.
Assuming you are using Internet Explorer 8:
Go to Tools then Internet Options
Under the General tab, in the Browsing history section click Delete.
Delete your history, cookies, and temporary Internet files.
Now go to the poker site and download their software again. Save the installation file somewhere new, like on your desktop or something like that.
Once you download them, right-mouse-click one and look at the properties and make sure the filse size is right. They'll be at least 10MB, not 25kb.
Try running them. If they work, great.
If they don't work, restart the computer in Safe Mode. (When you turn the computer on tap the F8 button and at the boot menu choose "safe mod") Once in safe mode, try installing them.