The HP tower is a strange mix of good an bad.
The amount of memory installed on this machine (the RAM) is listed at 512MB. This is not enough. It is barely enough to run Windows on it's own. Unless this machine is to be used for basic tasks (mine has an accounts system and is used as a printer server for other computers on a network) then you will need to upgrade the RAM memory to at least 1GB. This isn't expensive or difficult, just budget the upgrade into your price.
If you don't know about memory in computers, having too little will slow your computer down dramatically. This can be a quick machine, but with 512MB it can grind to a halt during intensive tasks. Startups can be excruciatingly slow. Upgrade the memory to solve all of this.
The machine has several USB ports which are mainly convenient. There are about three on the back and a couple at the front. The ones at the front are for easy access but they are set back into the casing a fair way and some items (like my MP3 player) will not physically fit because the MP3 bulk does not get past the casing. This can be solved by buying a USB extension cable, however. Once again this can be a little extra cost.
It does come with Windows XP Pro as standard, which is good and it is easy and simple to set up. A network card is standard and plugging this into a modem/router allows for instant access to the internet - assuming the router is set up.
There is no monitor with this computer, but on the plus side, this does allow you to pick a monitor you like, at the size you want.
Items supplied (or that I got) were basic. The CD-ROM on older models is not a CD-Writer, but newer models have the CD-Writer/DVD - so check that before buying a second hand one.
The mouse is basic. It does contain a scroll wheel but is an old fashioned ball type mouse and not a more modern optical. (Again this might not apply to later models).
I have had no problems whatsoever with this machine. It is an ideal business computer but you may need to upgrade the memory to get the best from it.