1st & foremost (since you didn't specify "Norton Antivirus" or "Norton Internet Security") antivirus alone will not protect one's networked PC. Having a firewall, (software firewall at least, software & hardware at best), is necessary to any real PC security effort. And for the record, Windows XP SP2's Firewall is insufficient in my opinion. That being said...
Norton Internet Security 05, to include Norton AntiVirus 05 & Norton Firewall, is admittedly very resource- (& wallet-) intensive . But when one looks at what NIS05 does, the sheer number of bases it covers versus the heavily-touted freeware suggested elsewhere in this topic, one begins to understand why.
I'll leave you to your own research, but after sifting thru reputable reviews by professionals to whom price is not the primary consideration, you'll likely discover the following: Other products, with (maybe) the exception of McAfee's & Computer Associates' suites of security programs, simply do not have the same expansive levels of protection offered by NIS05. While some products may "find more" so-called threats, as you will find in your research, many are found to be false-positives, the removal of which can have devestating effects on a system, when talking about OS/system files. Trusted commercial products have the budget & time-tested expertise to exhaustively test for & protect against this. Freeware developers typically do not have the personnel/income to achieve this.
Most Norton "haters" are more haters of spending the cash to buy the product (& subscription updates), more than the product itself. The number of such "haters" nearly tripled, I noticed, after product activation made it more difficult to steal Norton products using cracks & bogus serial numbers... coincidence? It also stands to reason that such a person might also tend to be stingy with system & hardware upgrades, thereby leaving them with yester-year's specifications in RAM & processor speeds - otherwise their resource concerns wouldn't be a concern at all. In my experience, even mid-to-low performing systems run great with NIS05, with a bit of tweaking. But pre-P4? 256MB of RAM? `Fraid you need an upgrade!
If your budget & system cannot handle Norton, then by all means, try a freebie. You'll get what you pay for, at least... sometimes suprisingly more, (ZoneLabs' ZoneAlarm Free Firewall is a fine example of excellent freeware security). But if you've invested in a higher-end system, I recommend investing a little more in a proven security solution to protect that investment.