Security Precautions

a) apply Windows Update regularly to block security weaknesses at source

b) use auto-updating anti-virus software (eg Norton Anti-Virus or McAfee) to monitor incoming and outgoing traffic and also set it to perform a system-wide scan, say, once a week
c) install a firewall - either in software (eg Windows XP's firewall, Norton or Zone Alarm) or better, in hardware (eg as in a typical broadband router). This is essential for Broadband.
d) don't open email attachments unless they are from a trusted source
 
ADVISABLE
 
e) If you are downloading software from the Internet, check that the site is legitimate by doing some research eg with Google
f) run spyware detection software (eg Ad-Aware or SpyBot Search and Destroy) occasionally to check that spyware has not found itself onto your PC
g) don't be fooled by 'Phishing' or 'Social Engineering' techniques designed to make you reveal your bank details, passwords or other personal information. No-one should ever ask you for this information
h) when going to a bank or other sensitive site, use your own link rather than one from an incoming email which might be directing you to a similar-looking one with a slightly different address.
i) turn off Windows Messenger which can generate annoying pop-ups in XP or Windows 2000 (Start>Control Panel>Administrative Tools>Services>Messenger>Disabled).
j) don't be tempted to 'unsubscribe' to spam email - this just tells them it's an active address and increases it's commercial value!
k) if you are being swamped by spam emails, either implement anti-spam software (eg Norton Anti-Spam) or move to an ISP with spam filtering or change you email address and don't make it public (eg on websites or message boards)
 
Once you have implemented these measures you are pretty secure from any form of attack so far invented. One slight annoyance with the firewall is that you may find that you can't access some sites or make some downloads with the firewall switched on, in which case you will have to switch it off temporarily.