New York--Online advertising spending, as measured on 300 Web sites, grew 80% to $359.4 million in the first quarter of 1999 compared with the same period last year, according to data just released by Intermedia Advertising Solutions, a unit of VNU. Microsoft Corp. topped the list by spending $8.8 million in the first quarter. IBM Corp. was second with $6.3 million spent in the quarter. Dell Computer Corp., Round Rock, Texas, moved into the top 10 by spending $2.6 million in the quarter, an increase of 2,786.9% over the first quarter of 1998. With total spending of $137.3 million, computers and software were by far the largest online advertising category, comprising 38.2% of total spending.