I found this very useful in converting DNA sequences.

http://www.vivo.colostate.edu/molkit/manip/index.html

Just input a DNA (or RNA) sequence and obtain its inverse, complement or inverse complement. The program can also be used to display a DNA sequence and its complement in double-stranded format.

or, write a simple script in perl:

#!/usr/bin/perl
#display an input sequence as the reverse complement, reverse, or complement or as double stranded

$opt = $ARGV[0] || die "usage $0 [rc,r,c,ds] sequence\n";
$seq = $ARGV[1] || die "usage $0 [rc,r,c,ds] sequence\n";
my $new;
if($opt eq 'rc'){
$new = reverse($seq);
$new =~ tr/ACTGNactgn/TGACNtgacn/;
}
elsif($opt eq 'r'){
$new = reverse($seq);
}
elsif($opt eq 'c' || $opt eq 'ds'){
$new = $seq;
$new =~ tr/ACTGNactgn/TGACNtgacn/;
}
else{
die "please use one of these options 'rc,c,r,ds'\n";
}
print "$seq\n" if $opt eq 'ds';
print "$new\n";

Can this script be used in window system or only in Unix system?
Thanks,