I'd stay away from hp since I know a few people whose motherboards have gone kaput when their machines were reasonably young, both laptops and towers. Dell is the best in terms of reliability and price. Macs are great too, even the minis.
I have a lenovo laptop now, and have to admit that it's not as sturdy as I remembered them to be. Love their full-sized keyboards, can't be beat! Don't have experience with their towers though.