We got a batch of Dell Vostro 1520's (30) about 6 months ago. From the offset they had an issue where they would occasionally freeze, but it wasn't being reported to me every time, so I got 1-2 reports a week rather than the 20 or so that were actually occurring....
When I realised how big an issue this was, I got on to dell support for assistance, they said there were no known issues with the model and asked me to monitor the situation. I thought this was tosh, so googled the issue and found that it was a known issue that many people were having. When running on battery, the laptops are fine. When running on power (with no battery), the laptops are fine. When running whilst the battery is charging, they freeze after between 2 and 30 minutes (seems worse when the battery is almost depleted).
So I got back on to Dell and told them I'd had a look around and that it was a known issue, 5 minutes later they came back to say "yes, it is a known issue... Here's the fix...". They sent me 3 files, one was the chipset drivers, one was the bios firmware and the other was a dell hotfix. I ran this on a few of the machines, but was still getting the issue. I got back to Dell and they said they'd investigate further. I also tested with a live boot CD at this point, and the machine froze whilst booting!
1 month later and Dell hadn't been back in touch, so I chased again (this was just after christmas). They agreed again that it was a known issue, but didn't offer a fix and said they were still investigating. At this point I gave them the ultimatum of either replacing the motherboards (googling suggested that everyone else with this problem had had their MB's replaced to resolve the issue) or taking back the dodgy kit for a full refund. 2 minutes later they agreed to replace all 30 motherboards, but couldn't give me a timescale as they have to outsource the motherboards.
Another month later and after contacting them every week for updates they still can't/won't give me a date!!!
Avoid the Vostro 1520's.