WASHINGTON — Retail sales of personal-computer software reached $1.3 billion in the fourth quarter of 1990, the Software Publishers Association reports. This was up 22 percent from 1989's last quarter. For the year, North American PC applications sales grew 26 percent. Desktop publishing was the leader, up 68 percent from the fourth quarter of 1989. Next came database sales, up 66 percent. MS-DOS sales grew 18 percent; Macintosh, 13 percent; and Windows applications, 198 percent.