I think Flock is pretty good as far as browsers go; it's a Mozilla-based browser so as far as security goes, it's not bad, and it's features make up for it's shortcomings. It's like the little sister of Firefox, so I wouldn't worry too much, Sanjida. But it's true that every browser is exploitable.
The problem we face (and this is especially true for anti-virus programs) is that we live in a cyberworld where attacks happen so quickly that the attacks can spread before patches are available. If you don't install those Windows updates immediately, you're vulnerable. Browsers and anti-viruses are really the last line of defence, not the first; the first line of defence is your behaviour. Don't go to suspicious sites; don't download things from unknown sources; don't click links in emails; don't open attachments; it's your behaviour that matters. Scans are great for a weekly or monthly check, but they're not going to do much more than tell you if anything has slipped through your defences.
All operating systems are vulnerable, but if you're know what you're doing they're not necessarily any less secure than any other. Windows is problematic but personally I think a locked down XP is more secure than Vista. There are no known Mac viruses in the wild, but the possibility for one exists; the same for Linux. It's just that 95% of computers use Windows; hackers can make money putting malware on your system, so why would they waste time targeting only 5% of the population?
And if you do get malware or a virus, you can try using a program to get rid of it, but a lot of them are nasty; they sink their teeth in so far that you risk damaging your system by removing them. SpyBot flatout refuses for some of them. That's why the only way you know you're safe is to reinstall, to go back to a previous safe version of your operating system. How else can you know it's really gone? That's why I think the install disk or at least a system restore disk is vital - forget an antivirus, that's what every user needs.
Hm, maybe this should be a post... isn't that what we normally tell people in the forums? :D