How to stop converting dollar amounts to dates?

I am running Office 2000 with XP Home Edn. Today Excel started doing something weird. If I try to enter a column of figures for dollar amounts, any set of digits from one to twelve followed by a decimal point and two digits for cents is automatically converted into a calendar date, even if the date is impossible (for example, 11.39 becomes 39 November).

I can prevent this by placing a "$" before each set of numerals, but if I do so, then AutoSum refuses to add them.

How do I prevent the autoconversion from occurring in the first place?

John H