Advice?

My erstwhile laptop (a Dell Inspiron 600m) may be dying. I have thought this before, and it has risen from the ashes, but it is nearly four years old, which is getting near the honorable age for retirement. It has been everything that one hopes for from a computer and more. In fact, it is more like typing on my best friend. I looked to eHarmony to find another perfect match, but apparently they really just have boys on the brain.

My reservations about this computer is that it is susceptible to viruses and the Microsoft programs wage war on other, often better, programs (here I am thinking of Firefox). In fact, iTunes runs into a startup conflict that I haven't been able to pinpoint, but which has made me shelve my iPod for the nonce. Would it be a good idea to move to an Apple laptop? The cheapest is $1099, whereas a decent Dell runs about $699 (the cheapest is $499—is the extra $200 necessary?).

Here are my criteria:

• Long-lived. I would like it to last at least four years, and to be reasonably compatible with new accessories (like iPods) at the end of those four years.
  
• Durable. I've dropped this laptop more times than I can say.
  
• Portable. I've taken this laptop everywhere and therefore used it more than any other computer I've had. I prefer a smaller, lighter computer to one with a high-quality but heavy wide-screen.
  
• What I actually use a computer for is the following:
  
  
  
  • Word processing and spreadsheets
    
  • E-mail and blogging
    
  • Music, videos and pictures (but I'm really not discerning about sound and video quality, and I like it that way because I can be happier spending less.)
    
  • I do like to be able to run a lot of programs at once.

As you can see, I clearly do not need something state-of-the-art. However, I do use it a lot, so probably don't want to go completely bare bones.