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Font size in IE keeps changing
Font Changes Size in Internet Explorer 5 Using Eudora Pro 4.1 or 4.2
http://support.microsoft.com/support/kb/articles/Q226/7/97.asp
Font Size Change Is Not Saved in Internet Explorer 5
http://support.microsoft.com/default.aspx?scid=kb;EN-US;q242984
Font Size Settings Not Retained in Internet Explorer and Outlook Express
http://support.microsoft.com/default.aspx?scid=kb;EN-US;q255062
If you are using Outlook 2000, go to tools -> options -> mail format -> fonts ->
international fonts and chance each font size entry to your preference.
If you do not use Eudora, and the other suggestions do not apply, perhaps
MSHTML.DLL is corrupt or not properly registered. Go to start / run and type:
regsvr32 mshtml.dll
If still no joy, use System File Checker (SFC.EXE) to extract a fresh
copy from c:\windows update setup files, or if using Windows XP, run MSCONFIG,
then click on the Expand File button.