Good stuff. Not sure why it happens but it does occasionally. We took a broad hit at clearing this time. Next time it happens try clicking the Cache box on its own, if that works then you shouldn't have to retype your history. If it doesn't work then click the rest as above and retry.
Another speed up for browsing is to stop Flash loading automatically. The last couple of release of Flash on the Mac have been worse than usual and have stated causing lots of browser crashes. This can be stopped by loading Flashblock on Firefox and ClicktoFlash on Safari. These stop the autoload but give you an option to click the box where flash would have loaded to run the flash movie etc. this really speeds up page loads.
For virus malware protection, I mentioned not using Norton earlier. This is because it gets in the way on a mac and is really hard to remove. Symantec is Ok, I use Intego. A good free version is ClamXAV. There are still no viruses that attack mac OSX systems, but there have been a couple of cases of Trojans being released in pirated Apple software, so worth using a checker in the backgrould