1) You can't change Celeron M chip and put other cpu unit you have to get a new motherbaord it is like full new process so you need to buy new computer. if you are interested good computer with good price you can get re certified dells.
2) You can get some laptops ubuntu installed.
http://www.dell.com/content/topics/segtopic.aspx/linux_3x?c=us&cs=19&l=en&s=dhsdell has a page for ubuntu computers.
3) You can buy Macintosh 1000$ no problem but decent one's start from 1200$ Macintosh is good for basic usage like Email, browsing, word,excel but if you have many customers with windows machine don't get a mac. Problems between sending documents from mac's and windows common. they don't work perfect with each other. it generally works but there is always one problem every month.
I work setup so many cross-platform environments and we have to do many specific settings so mac and windows works with each other without a problem.
I think if you buy windows 7 with quality anti-virus you will not have any problems with windows. What happens with pcs are people buy it and they won't buy anti-virus so they always have problems.
Actually I don't know why Google change it because most famous hacker said it is easier to hack mac then windows 7.
"Mac OS X is like living in a farmhouse in the country with no locks, and Windows is living in a house with bars on the windows in the bad part of town."
Charlie Miller, Apple Security expert
SO actually mac's are not safe but there not much people interested to hack them. if Google changes to mac, I'm sure some hackers will be interested to hack mac's. :)