Home computers
This is what I would recommend to run on a single or stand-alone home computer:
Dell Dimension line
Windows XP with SP2
A UPS is preferred, although a line conditioner will suffice. Note that if your power is not stable (brownouts or undervoltage conditions), get a line-conditioning UPS (hint: Triplite has them for under $40).
UPS 1st Choice: Triplite
UPS 2nd Choice: APC
Automatic updates enabled to install all updates on Wednesday's 3:00 am.
Microsoft Office 2003. Make sure to get Word, Excel, Outlook, Publisher, there are different editions.
Antivirus: Norton Internet Security™ 2005 AntiSpyware Edition
A minimum of one external hard disk for backup. Maxtor's OneTouch II has a very nice feature set. I also like Western Digital's products just as well. I encourage you to get two so they can be rotated.
Additional considerations
I have run the below software titles for many years and been very pleased with them. You might consider them.
Antivirus Grisoft AVG 7.0 Professional Edition.
K9 (Bayesian spam E-Mail control).
Purchase two AMS Venus enclosures and two hard drives. Then use the built-in Windows program NTBACKUP and schedule a nightly backup job. Swap (rotate) the enclosures every few weeks. I have done this with and it's much less expensive than the pre-built external hard disks. But you do have to build it and create the backup jobs.
Google Toolbar (popup blocker with no privacy issues)
Please don't put your computer in direct sunlight or where the heater vent blows or next to a space heater. Heat is bad on computers!
If you do not have Windows 2000 or XP, please purchase Windows XP. Please leave Windows 9x and ME behind.
Avoid these
Please do not purchase any of these:
McAfee
Computer Associates