There have been 20 security advisories / issues for IE7 so far.
Currently, 40% (8 out of 20) are marked as Unpatched with the most
severe being rated Highly critical.

http://secunia.com/product/12366/?task=advisories

There have been 121 security advisories / issues for IE6.x, 21 of them
(17%) are considered as unpatched, with the most severe being rated as
moderately critical.

http://secunia.com/product/11/?task=advisories

IE7 is not necessarily a better or less vulnerable browser when
compared to IE6.

IE6 is still supported by Micro$oft because although they still
support Windows-2K, they have handicapped 2K by preventing IE7 from
being installed on it.  So they must still support IE6 as long as they
support Win-2K.

Some of the IE6 security patches released for Win-2K work just fine
when transplanted to win-98 machines, so don't necessarily consider
win-98 / IE6 to be at risk at this point in time.  These tricks have
been talked about in the newsgroup
miscrosoft.public.win98.gen_discussion, as well as the win-9x forum on
msfn.org.

It's a good idea to do the following (on any system, win-98, 2K, XP,
etc):

1) make sure you have a NAT-router between your broadband modem and
your computer.

2) use Spybot SD and Spyware blaster to "innoculate" your broswer (IE
and/or Firefox).

3) Use a hosts file

4) Uninstall ALL old versions of your Java runtime engine (JRE) and
install only the newest version (available from Sun).

5) Don't waste your time obtaining or running a software-based
firewall.

6) Antivirus programs are becoming passe.  Place more importance on
software that monitors your registry in real time and can alert you to
changes being made without your knowledge or permission (most viruses
and trojans will try to modify key settings in your registry during
the infection process).

7) Take heart in the knowledge that modern viruses and trojans are
written to function correctly on NT-based systems like 2K and XP and
will probably not run correctly (or with the intended result) when
exposed to a machine running win-98.

I use Firefox and Opera, and almost never IE.  Opera is for daily driving, and
Firefox I use for those sites that Opera has a problem with, such as income tax
filing.

"Secunia has issued a total of 13 Secunia advisories in 2007 for Microsoft Internet
Explorer 6.x. Currently, 31% (4 out of 13) are marked as Unpatched with the most
severe being rated Less critical"
http://secunia.com/product/11/?task=advisories_2007

"Secunia has issued a total of 16 Secunia advisories in 2007 for Microsoft Internet
Explorer 7.x. Currently, 31% (5 out of 16) are marked as Unpatched with the most
severe being rated Highly critical"
http://secunia.com/product/12366/?task=advisories_2007

The still unpatched Highly critical advisory for IE7 (does not affect IE6):
http://secunia.com/advisories/26201/

"Secunia has issued a total of 12 Secunia advisories in 2007 for Mozilla Firefox
2.0.x. Currently, 17% (2 out of 12) are marked as Unpatched with the most severe
being rated Not critical"
http://secunia.com/product/12434/?task=advisories_2007

"Secunia has issued a total of 8 Secunia advisories in 2007 for Opera 9.x.
Currently, 0% (0 out of 8) are marked as Unpatched"
http://secunia.com/product/10615/?task=advisories_2007

You be the judge......