Long story, short version: As long as Windows (XP or Vista) is fully-patched at Windows Update, IE7 is secure.
Long story, longer version: IE8 (and IE9) include enhanced security so they're even
more secure. See...
versus...
Note: An ever-increasing number of websites no longer support (or fully-support) IE7.
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[1] IE9 is available for Windows Vista only.
~Robear Dyer (PA Bear) ~ MS MVP (IE, Mail, Security, Windows & Update Services) since 2002 ~ Disclaimer: MS MVPs neither represent nor work for Microsoft