admin powers on home network?

This belongs in the TQ&A section, but I'll cut you a break since you're obviously a newbie here.

If you know the administrator password on all the machines, you can login and change the access rights for everybody on the network to users or powerusers, HOWEVER, this will make it so that everytime they buy a new game, or want to download music, YOU'RE going to have to login and install it for them... BIG PITA!

Your BEST BET is to install firewall, antivirus, and adware scanners (I reccomend both Spybot and Ad-aware) on all computers, and educate your sisters on what is and is NOT safe internet activity.

One thing you may want to do is install a Content filtering software. I have not messed with any of these, although a collegue has installed something called Dan's Guardian. This is a LINUX based program. You would install this before al of the computers and after the Internet connection. Meaning all connections going through the LINUX box would be checked for proper sites etc... I could remble on alot about nothing, but I will let you research it for yourself. The site to download and read up on this is http://dansguardian.org/. Have fun....

You stated your computers are windows ME windows 98 and XP home. Therefore you can't set their user rights to their own computer unless they are on XP. For windows 98 and Me you would have to use group profiles locally on that computer. Group profiles makes it so you can not change things like the screen saver and cursors. I think it might have enough power to prevent users from installing programs. This is all handled locally unless you install an actual server, I can't see this as being a program people would mess with.

As proven many times, the largest risk you can take is to have uneducated personnel. Educating your sisters is your best bet. Otherwise, every time there is a new virus, spyware, adware, etc. you'll have to clean the machines.

As is famous in the security field, the largest threat is the one that comes from the inside, especially by uneducated people.

Rule of thumb: Education, Education, Education...

In addition to Ad-Aware and SpyBot, I would recommend using SpywareBlaster and Microsoft AntiSpyware.
SpywareBlaster: www.javacoolsoftware.com/spywareblaster.html
MS AntiSpyware: www.microsoft.com/athome/security/spyware/software/default.mspx

Education is the best bet, that, or refuse to fix the machines for them when they blow them up, let them have to fix it themselves one time, and you will probably never have an issue with them downloading garbage again.

After trying to help the Tech Coordinator at my high school, I've come to the conclusion that stupidity can't be cured, only limited - what needs to be done on your local network, other than purchasing a firewall and blocking certain content that you don't want to pass into it, is to try and *prevent* the majority of the trash that comes into your environment. Micrsoft AntiSpyware is decent, but Ad-Aware SE and Spybot S&D are both very effective tools. With even the free version of Spybot, you can immunize each system from accepting a certain amount of junk, and with Ad-Aware Pro you can schedule automatic cleanups. Norton 05 and Internet Security would be even more powerful tools, each set on a nightly cleanup schedule, but the biggest problem you face, financially, would be to purchase Windows XP with Service Pack 2. I've never been a fan of the new service pack, don't get me wrong, but as far as its activex and pop-up blocking capabilities, it's unmatched. You can set restore points to go back to (much better than your current ME, Migraine Edition), and it's an overall more stable operating system. Good luck with the prevention, worst come to worst, you can always yank the RJ-45 from the hub. ;)