I'd say go for it.
As said, the low-end area's will be fairly quiet, but the storyline (especially in NF/prophecies) is great IMO, which will keep you entertained if you don't skip cutscenes, and there are still the wandering people that are either
1) Levelling a new character, and willing to come along with you
2) Working on the title to complete each and every mission with bonus rewards. These people will likely be level 20, so will help make the mission a breeze, although at the cost that you won't be learning as much
3) Helping a friend, and have space to help you too.
As many have said, you can download it on as many computers as you like, but the account can only be logged in to one character at any given time.
http://guildwars.wikia.com/wiki/Main_Page is a great helping hand for a new player, gives help on quests and missions, weapon details etc, and its sister site,
http://pvx.wikia.com/wiki/Main_Page will give a great deal of information on what builds (Sets of 8 skills/party members) are good, although I would actually suggest ignoring PvX until you are level 20 and looking to get into hard mode content, and instead take the normal mode at your own pace for each campaign.
If you need any more info, feel free to drop me a whisper in-game

Just add 'Robin Hood UK' to your friends, and you should be able to see me regardless of what character im on as.