I'd have to agree that the largest driver would be how portable you want it to be. As mentioned above, the main components that tend to go kaput are actually pretty easy to replace on laptops these days (Hard Drive, RAM, LCD screen, Keyboard). Other than that I'd say the biggest reason for needing repairs is all virtual... Basically, viruses, malware and the like, which can be cleaned and or prevented with some minor computer knowledge and some google searching. Also, if you're a little more savvy, 150 bucks on a large external hard drive will allow you to back the system up well enough that restoring it would be a snap if it did encounter a problem.
If it's an option, regardless of Desktop vs Laptop, I would HIGHLY recommend that if you go with a major manufacturer (Dell/HP/etc), pick up something from the business line vs the consumer line. A comparison would be if you had an IBM ThinkPad vs the model you have now, the components are just better in the business grade machines and don't fail as often or as quickly. A typical retirement schedule for a machine running in a corporate type office can be as far out as 5 years if the equipment is maintained and the user's work doesn't require high end computing (read AutoCAD, Engineering, Computation, Graphics Design).
I've been a SysAdmin for many years, and the consumer grade equipment is always below par compared to the business level stuff. Also, many employers who use Dell/HP/etc at the office will have an employee purchase program that will allow you access to the business grade equipment as well as provide a minor discount (typically 8-10%).
Lastly, investing a little in a good machine will ensure that the system lasts a while. I've been running the same little laptop (entry level MacBook) for email, internet and minor personal things since 2004 and still haven't outgrown it from a computing perspective. I know Mac vs PC is as much of a debate as Maple vs Rosewood or rounds vs flats, so let's not go there.

You could definitely get the same life out of a well maintained PC. Trick is that you have to spend a little time and money on running regular maintenance routines. Currently, Macs are a little less susceptible to viral issues due to them not being as widely used (another debate for another forum). Also, keep in mind that there is about to be a 64-bit revolution in computing and nearly all 32-bit hardware will start to become outdated soon. So you may end up with a new 32-bit system that you have trouble loading apps on a couple years down the line.
Another easy one would be to upgrade the RAM in your current machine. Depending on which version of windows you're running (assuming it's a windows box), you could easily go to 2-3GB on a WinXP machine and can go further with Win7 (I wouldn't suggest vista). This would cost you at most $200, depending on where you got the RAM from.
Fee free to PM if you find something specific that you want some feedback on.