There is no NEC requirement for how high most receptacles should be mounted, but 12" is common. I'd just be consistant with the rest of the house. If the window won't permit consistancy or it looks odd, just use a convenient height. Code requires that "... receptacles shall be installed so that no point along the floor line in any wall space is more 6 feet, measured horizontally, from an outlet in that space, includeing any wall space 2 feet or more in witdth ...". I.e. 12' is the maximmum spacing; you can, of course, make them closer. Code also requires "Receptacle outlets shall, insofar as practicable, be spaced equal distances apart." Don't put receptacles above baseboard heaters if you will have any. You really should get a book on basic wiring before doing this job; there are several small but important details to be observed.