window function help

Hi,

I'm trying to figure out how to count the number of rows within a fixed range of the current row value.  My table looks
likethis: 

SELECT chr_pos
FROM mutations_crosstab_9615_99
WHERE bta = 38
LIMIT 10

chr_pos
138
140
163
174
187
187
188
208
210
213

chr_pos is integer and represents the base pair position along a chromosome.

It looks to me like a window function would be appropriate but I cannot figure out the correct syntax.  What I want to
dois count the number of rows within +/- 20 of chr_pos (the current row).  Given the above example, for chr_pos = 138 I
wantthe count of rows between 118 and 158.  For chr_pos 187 I want the count of rows between 167 and 207 etc for all
rows. The result I'm looking for should look like the following: 

chr_pos,num_variants
138,2
140,2
163,2
174,4
187,4
187,4
188,4
208,6
210,3
213,1

Is there a way to do this with a window function? Any help would be appreciated.

Thanks
Bob