Mine only last about 18 months, but my uses aren't typical. Gaming on laptops gets expensive.

That was a very slow machine even when new.

Yes, you should upgrade.

There are plenty of 3-year old machines that would not be worth upgrading today, at least from a speed perspective if you don't play video games or do particular sorts of professional work that need all the speed you can get (architecture, mechanical or structural engineering, some kinds of software development work, etc.)

A mid-range business laptop back to about 4 years ago (with say i5-520m processor, say) is going to be fast enough for just about any "general use" these days, and a mid-range business laptop from late 2006 (with say a Core 2 T7200) will be sort of the minimum tolerable speed. The latter probably came with a gig or two of memory, and really needs to be upgraded to the maximum it'll take (4gb, with 3.5gb usable) to still be worth keeping around.

Compared to either of the above, a brand new midrange or even low-end business laptop will mainly be lighter, run cooler, and have better battery life; it'll be much faster than the 2006 model, but so will the 2010 version, and the speed difference between the 2010 one and a brand new one won't be that noticeable to most people (assuming it doesn't have an ultrabook processor, which may be not much faster than the 2010 version even on paper.)

There are some very good refurbished deals on 2010 and 2011 laptops these days; the Lenovo T410/T410s/T420/T420s/X201/X220 are all great machines that can be had refurbished for the price of a new low-end piece of junk.

With 8gb of memory and the hard drive replaced with an SSD, virtually anyone will be happy with one of these for general use. I still use my X201 (with 8gb and an old Intel X25M SSD, hardly a barn burner) day to day to carry to meetings and on short trips when I don't need to do coding (or can do the coding remotely) and when I don't expect to have time for gaming.

I found the N280-based system I got (very cheaply) in mid-2009 -- which was itself a step faster than the Z530 -- too slow when it was brand new; it bogged down on Javascript-heavy websites by the much more limited standards of the time, let alone the current generation of them. It was just tolerable with Office 2003 and with Office 2007 except for whatever it was that changed in Outlook.

Viruses are readily avoidable.

XP and older Microsoft OSes tended to have corruption issues requiring re-installation eventually; Vista (for the few of us that used it) and newer mostly fix that... although eventually you'll want to reinstall. My wife's laptop just hit the 4 year mark and it's starting to show its age. The specs are still quite good (i5-540m, 8gb, 300gb Intel 320 SSD), and while I've been encouraging her to let me get her a new one (she's been complaining about the weight for ages), I suspect that a clean installation of Windows 7 would leave it feeling quite perky.

Now, physical quality of hardware varies a lot on the PC side, and a lot depends on what you value; higher-end business laptops on the PC side until very recently tended to be built like tanks... super durable (more so IMO than Macs) but heavy and bulky.

Cleaning out cooling often is a BIG help. Depending on the model, how easy it is get at the cooler can vary a lot -- as is the orientation of the fan and heat sink. Some of the worse designs end up basically with a felt of hair and dust stuck in between the fan and the heat sink and no way to remove it without major disassembly.