Hey Jane

I can't tell by reading all the responses here whether your 3 questions have been answered. If not, just ask again.
I haven't experienced these popups myself, nor use that e-card website, but 5 different computers in the family circle all ended up with WinFixer infections. I always wondered
how did they get this stuff? They were also using IE at the time.
In the link Corrine provided
here, if you scroll down to:
Hijack Number 2 at 123greetings.com - the popupThere are 7 pictures. Seven persistent screens! to go through to get away from this crud!?

In the pics I can see the yellow protection bar based on the IE browser settings at work...sooo, either my family's browser settings were below the default level for the Internet zone, and/or they clicked on the "OK" or "Cancel" buttons, instead of the X (close) at the top right corner.
Quite informative for me to see "it" in action.
Packard Bell (EasyNote) + Windows Vista Home Premium 32 bit +Windows Defender + Norton Internet Security 2007 (90 day trail) - Pre Installed.
I accept only a few needed 1. part cookies (Never 3. parts) and also very few pop-op are alloud.
Have installed Spybot, Search & Destroy + SpywareBlaster + SUPERAntiSpyware Free Edition.
Use Firefox and Thunderbird as Standard programs. (Internet Explorer and Windows Mail is seldom used)
Everything is up-dated.
This looks like you have made good choices for "layered" protection. Others might have additional ideas.
When I reorganised the XP, blocked 3. parts cookies and pop-op it was normal again.
Cookie settings are in your browser(s).
I do not accept 3 party cookies either, in IE or Fx. Have you found that setting in Firefox...no 3rd party cookies?
And now also McAfee SiteAdvisor (By the way - I Googled: "123greetings", and a long list came, where a few was Green - But I believe they are just as bad as the red - Guess there will come pop-op, as it are not Wikipedia or tecnical explanations, but advertising) I would not try !
That's why members are providing you with safer e-card sites.
You are on the right track with your learning

Keep asking questions. As you become more familiar with the information from these layers of protection (programs), you'll be able to make smarter choices online and hopefully avoid these infections. If not, you know where you can come for assistance.