Dell has been found guilty of breaking advertising rules for broadcast and national press promotions.
The Advertising Standards Authority (ASA) ruled against the computer manufacturer over two printed adverts and three television commercials, although Dell was cleared of a complaint levelled at another of its TV adverts.
Three viewers and one reader complained that one set of ads was misleading because they could not purchase the computers at the advertised prices.
The problem occurred because Dell added support costs to the price during the online purchase process.
While the ASA noted that pre-selection of service options is common practice in the industry, it felt that Dell's customers could only get the advertised prices if they were aware of this practice.
Dell changed its website to include the phrase: 'To reach the advertised price please select a downgraded service from Support Services options below and update price.'
"Nevertheless, the ASA concluded that, because earlier versions of the advisory text on the website were not sufficiently clear about how to obtain the advertised price, and because the need to de-select the automatic upgrade was not made clear in the ads so that customers wanting to purchase the items could do so without undue difficulty, the ads were misleading," said the ruling.
In the second case, two complainants tried to take up an offer of a 'One Year Euro Collect and Return Service' which Dell said had been included in the ads by mistake.
Dell claimed that the text should have read: '90 day Collect and Return service', but offered to honour the original promise when it realised its mistake.
"The ASA noted that the claim 'One Year Euro Collect and Return Service' appeared in error," the ASA's ruling said.
"However, we were concerned that the error was repeated in several ads during the course of the month and that in at least two cases Dell had not honoured the promise when asked to do so by customers."
The watchdog concluded that the ad was misleading and asked Dell to take greater care when preparing future similar ads.
Dell won its case against a complaint about a Dimension desktop that was advertised with a 17in flat panel monitor for £289.
The complainant claimed to have looked for the product online but could find only a 15in monitor at the advertised price.
The ASA found in Dell's favour and said it was satisfied that the 17in Flat Panel Dimension 1100 was available at the advertised price during the promotional period.
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