IBM ThinkPad's Compaq Evo's Dell Latitude's are the commercial grade for these major computer companies.
High end P-3 are as good as some P-4 systems.
Large amounts of Ram or large hard drives not needed for the average PC user, however upgrades are available to meet your needs.
Windows XP is compatible with 99% of the software on the market.
Vista has never been very stable and is much slower.

14"-17"screens 20-100 gig drives 512-1 gig ram DVD or CDRW  XP Pro and home
Priced as low as $200.00 with 90 day warranty