I have two Lenovo N500 laptops on my desk next to me right now - one is downgraded to XP (running natively) and the other is upgraded to Windows 7 Pro from the awful Vista they shipped with.
Both have been excellent performers.
At last count I had close to 200 Lenovo in various client networks, with few issues to speak of (mostly user error

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Windows 7 is the best thing Microsoft have done in an age - responsive, stable, it even looks pretty good (if that honks your hooter) and many older programs work very well in compatibility mode without having to emulate XP by using the Windows XP Mode download, which only works with Windows 7 Professional, Ultimate, and Enterprise incidentally.
Compatibility mode on Windows 7 has allowed me to run almost every application I wanted to that I didn't want to upgrade, and XP Mode easily managed to run those apps compatibility mode couldn't cope with which, if memory serves, was one HP photo scanner and some medical haemostasis instrument software (don't ask

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I get a little irritated at the fundamental changes in Windows 7 from a sysadmins perspective where you sometimes have to go looking for stuff that you're used to finding elsewhere, but there's no denying its stability. Security is good too, with a very capable and highly configurable software firewall built in.
I recommend to most Windows 7 users to download and install the free (for home users) and excellent Windows 7 Firewall Control which makes configuring the firewall a breeze for non-techies in terms of granting or denying application rights.
But dragging this kicking and screaming back to the original question, Lenovo is as good as anything else - it all comes down to the right combination of price and specification at the time of purchase. A friend of mine runs some massive networks and one in particular is almost entirely running on Acer kit - he got a very impressive spec for the price he paid and extended warranties (original 1 year plus 2) were well under a hundred quid per unit. Not a recommendation so much as an observation.
My opinion of Dell is very divided - throw quarter of a million quid their way and you get incredible customer service. Several friends and one family member have had bad experiences with after-market service from Dell on personal purchases, but as in all things you only ever hear the whines rather than the successes. I no longer use them after they spectacularly failed to deliver kit I'd ordered well in advance for a large network infrastructure installation - two days before I was due to receive the kit I called them to find out what was going on to be told they'd almost started building it
I ended up buying off the peg HP stuff instead and having it overnighted, and then wrangled with Dell for weeks about the order cancellation and refund
Like I say, you only ever hear about the failures; never the successes.
Good luck whatever you end up with
