There are dozens (hundreds?) of ways to solve problems - from simple to complex, easy to hard, etc. Whether you write a function or use SQL code, you need to use the ASCII characters for carriage return (chr(13)) and linefeed (chr(10)) in that order to create a linefeed in the Access cell. Otherwise, the characters appear as "little squares."
I apologize I didn't read the original carefully enough. You can use Replace in Access 2000.
Update tbl Set col=replace(col,"str",chr(13) & chr(10))
Where col like "*str*"
If you don't have Access 2000 you can write a function as has been suggested and use that function in an update query or you can use the following update query.
Update tbl Set col = Left$([col],InStr([col],"str"

-1) & Chr(13) & Chr(10) & Right([col],Len([col])-InStr([col]," "

+1)
Where col like "*str*" Terry
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