The obvious malware/spyware issues need to be dealt with on any computer and for this I suggest reading the TIPS on OPTIMIZING and then install the free utilities available through the SOFTWARE/SUPPORT page. But there are many other things you can be doing from a web point of view.
Firstly, if you have been retrieving your mail through an email program such as outlook express, outlook, thunderbird or other, try using the web-based email. Most internet providers provide this through the use of a web browser, logging in to your email account. An example in Canada for Telus would be http://webmail.telus.net and for Shaw it would be http://webmail.shaw.ca
You can try "webmail." whatever your email domain is and might get their directly but you can check with your internet provider to be sure. Next, because you are now using the web browser to get your email, you should be using a FAST browser and for that, you can't get any faster (currently) then the Google Chrome browser with Firefox a close second and Safari as well, although this is more prominent with Mac machines.
If you happen to be using the other Google products like GMAIL then you will truly enjoy how Chrome works. If this is to be any indication as to where the Google operating system will be in the internet marketplace, I believe they will be a true competitor to the current Microsoft position in the marketplace.
Finally, with the May release of the Windows 7 Beta Public Release Candidate being made available to everyone, and the fact that it will continue to work until next June 1, 2010, there is no better time for an old machine to upgrade, since the biggest hardware upgrade you will need to make is making sure you have a minimum of 512mb of memory and a 1 ghz processor. This is a great way to get your system up to date with all the latest available security patches and updates, by getting the latest operating system, especially since Windows XP will soon be out of product support by Microsoft. And, since you will have until next June to actually purchase the new operating system, you might find that you simply end up upgrading your computer next June and not even buying the operating system since you would be buying an entirely new computer, one that comes with Windows 7 installed... it's just a thought, and certain one that can have you enjoying the increased internet speeds. Oh, and if you haven't already contacted your internet provider to take advantage of the increased speeds they offer, I highly suggest it, as the typical $5-$10 increase in your monthly bill you will find your productivity on the internet increases as you have the ability to multi-task at higher speeds!