John Robertson, an education lecturer, carried out a study for the Scottish Office of three schools in Dumfries and Galloway - Maxwelltown High, Lochside Primary and Mouswald Primary - which piloted the scheme.
He found they were a useful partial strategy but heads still needed to be able to buy new machines. The warning follows an announcement by Helen Liddell, the Education Minister, of a national drive to encourage firms to donate their old computers to schools.
Mr Robertson's study assessed the use of recycled PCs in the three schools and discovered a variety of responses, with the primaries having a more positive attitude.
Although recycled machines were useful as they gave pupils computer access and were adequate for word processing, Mr Robertson said 486s were not fast enough if schools were to make use of the National Grid for Learning.
Maxwelltown High found extra cash during the project and opted for new computers.
The Scottish School Board Association, which is setting up a parallel scheme and remains critical of the Government's route, however said the pilot scheme had shown "a very positive reaction".
"Both teachers and pupils have been without exception satisfied, if not highly satisfied, with the quality of the systems. The result of this pilot has shown that there is a place for refurbished computers as well as new computers in the education system in Scotland," it says.
But Mrs Liddell has appointed Professor Nigel Paine, chief executive of the Scottish Council for Educational Technology, to head a working group on refurbished computers and asked the school board association to join.
The minister said: "SCET's involvement will ensure that refurbished computers are of a specification and quality which will make them a useful addition to the Grid."
The Royal Bank of Scotland pledged 1,000 computers in the next year. Lord Younger of Leckie, the bank's chairman, said: "We will be paying for the additional cost of refurbishing and providing a three-year warranty for over a third of them. We will ensure schools in deprived areas will receive these computers free."
The Scottish Office says a study in the Netherlands and United States has shown that machines must be refurbished to an agreed minimum specification; adequate guarantees and support must be in place; authorities and schools must have proper IT strategies; and the cost of machines must be low enough to make them an economic proposition over their life.
Other members of the SCET working group include the Association of Directors of Education, Scottish Business in the Community, Scottish Enterprise and Sara Dodd, a consultant.