AVR-Chat

When I use the AVR's internal oscillator or an external crystal
oscillator, like 1.0 MHz, is the frequency 1,000,000 Hz (give or take
a few Hz for accuracy, temp, etc)?  And not a power of 2, like 2 to
the 20th = 1,048,576?

The reason I'm asking, is because the prescaler divisors are all
powers of 2 (i.e., 2, 4, 8, 16, 32, etc), and this makes it difficult
to use the prescaler to get whole number frequencies.  For instance,
if I use a 1 MHz clock and the 1024 prescaler, it gives an interval of
0.001024 seconds instead of a nice, even number like 0.001 seconds.

I guess my real question is, "Given a particular input clock, how can
I make a timer that counts in, say, one millisecond intervals"?  And
how can I keep it as accurate as possible over a long period of time?

Thanks for any help.

Jeff