Ad-Aware and Spybot-Search and Destroy are not very happy about Microsoft's data mining and personal data transmission policies.
For instance, Windows Media Player is considered a "medium" threat by some software, since it reports your usage patterns back to Microsoft, and likely doesn't encrypt this transmission across the internet.
If your homepage IS indeed automatically going back to MSN without you doing it...hmmm...what kind of computer and ISP do you use? I know a lot of software now is viral--for one example, AOL "infects" your computer by changing all of your preferences and such. One guess could be if you got a big rebate or something for signing up for MSN service for a year, it might require (force) you to use MSN as your homepage. Or you might be using an MSN service that requires this.
Unless you detect some sort of suspicious activity, you should be okay. Plus, you have dial-up, and the chances of a hacker using your box as a proxy IP or whatnot are VERY unlikely, due to your limited bandwidth.
I would do this, though, since I am paranoid:
1. Do a "port scan" There are many free sites online that will do this. This lets you know how many doors are open to the public. Following this, close the open ports.
2. Install a firewall that allows you to monitor traffic and running processes...then you can easily see what is going where, and who is requesting it.
3. D/L, install, and run Spybot Search and Destroy. Its a free program, and this plus Ad-Aware and you're all set.
4. If you visit porn, MP3, bit torrent, or password sites, it is a VERY good idea to get an IP cloaking program and an advanced pop-up blocker, and a registry mod shield.
But...you probably don't need to do these things
