I have been trialing, on Win98SE BrdBnd, Firefox now Ver 1.0 and Opera Ver 7.6(beta) browsers.
I am no expert but I find both of them excellent .... I believe they have very few vulnerabilities.
If any one feels a little concerned about 'messing up the system' then do not worry, neither 'take over' in any way and sit quite happily beside IE6 etc.
IE is required for M$ auto / by tool updates.
Opera is the mighty one where almost every facet may be tuned to the user's desire, just about the fastest too.
Firefox is lighter in weight but has a nice feel to it ... both work fine with web banking etc and it would appear are pretty secure.
Since starting to use both I have had no spyware or glitches.
Some sites do not open correctly, not many, It is said that the sites themselves are to blame not the software ... ?
Both have excellent download managers ... especially Opera, with resume download etc ... useful if link lost before complete.
P