This will be easy for a VB coder: I have a row range E1:DV1 with concurrent
dates starting Aug 2010 to Nov 2010. I want the rows (lets say 20) below
weekends to be autofilled (Result: Weekends will be visible as columns)
without being affected by subsequent conditional formatting or VB code. The
subsequent code or conditional formatting must autofill between date ranges
obtained from corresponding cells in columns C&D.

Lets see if I fully understand your question.
You have a row of dates (1st Aug 2010 to 30th Nov 2010) in cells E1:DV1.
You have a start date in column C
You have an end date in column D
You want all weekend dates filled in for all the columns.
You want the weekday dates filled in for the period between the start and end.

If above is correct then it can be done with a fomula without using VBA.

Assume that C2 contains a start date and D2 an end date.
Enter the following formula in E2.

=IF(AND(E$1=$C2,E$1<=$D2,WEEKDAY(E$1,2)<6),E$1,IF (WEEKDAY(E$1,2)5,E$1,""))

Note: Although the formula may appear as 2 lines in the post, it is actually
one line so you will have to fix that up when you copy it into the formular
bar.

You should be able to copy the formula across all columns and down as far as
you want.

Entering or changing values in columns C and D should produce the results
you desire.

Note: If both C and D are blank or only C contains a date, only the weekends
will be populated. If only column D contains a date and C is blank, then all
columns will be populated.

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Regards,

OssieMac