You don't need any 3rd party antivirus app to keep your Mac malware-free. Macs are not immune to malware, but no true viruses exist in the wild that can run on Mac OS X, and there never have been any since it was released 10 years ago. You cannot infect your Mac simply by visiting a website, unzipping a file, opening an email attachment or clicking on a web link. The only malware in the wild that can affect Mac OS X is a handful of trojans, which cannot infect your Mac unless you actively install them, and they can be easily avoided with some basic education, common sense and care in what software you install. Also, Mac OS X Snow Leopard and Lion have anti-malware protection built in, further reducing the need for 3rd party antivirus apps.
If you want to scan for Windows malware without the increased vulnerability, ClamXav is a good choice, since it isn't a resource hog, detects both Mac and Windows malware and doesn't run with elevated privileges. You can run scans when you choose, rather than leaving it running all the time, slowing your system.
Not true. ClamXav takes very little in the way of resources and RAM for periodic scans. You may be thinking of the Sentry feature, which isn't required.
To get a complete picture, launch Activity Monitor and change "My Processes" at the top to "All Processes", then click on the CPU column heading once or twice, so the arrow points downward (highest values on top). Then see what apps are using system resources.
It looks like it's performing the initial scan of your system, which makes sense since you just installed it. Naturally, that would take far more resources than monitoring or scanning after the initial scan is complete.
