- I don't believe hard drives slow down as a result of wear and tear/age.
- There are a number of benchmarking programs to measure performance. If your HD has been benchmarked by tom's hardware, you can compare it to their data at http://www.tomshardware.com/charts/hard-drives,3.html.
- I understand drivers can make a difference, though I've never noticed anything. Ensure that you have the latest drivers and BIOS for your motherboard.
Also, harrymc mentions starting in safe mode to see the difference in speed. I'd be careful here, because safe mode disables the os file caching, which might negate some of the effects you'd otherwise see.
A good program to help you prune your startup list is autoruns, available at http://technet.microsoft.com/en-us/sysinternals/bb963902.aspx. Don't disable anything you might need, but you might well be able to disable a number of things. Remember that stopping something from starting automatically doesn't prevent you from running it when you want. Personally I always remove any quick starters and update schedulers, amongst others.
You might also want to check out a prog called mydefrag (was jkdefrag) at http://www.mydefrag.com/ which uses a fundamentally different approach to defragmentation than Windows, and is supposed to improve disk speeds.
mydefrag works by moving files towards the start of the HD where the speeds are typically faster (check it with HD Tune). Because of this, I create a 20 gig first partition for Windows and software, and store my data in the second partition. This should help keep boot speeds more consistent.
Finally, I've also heard it can be useful to empty your prefetch: instructions at http://www.windowsnetworking.com/articles_tutorials/Gaining-Speed-Empty-Prefetch-XP.html.
Hope that helps,
PS A number of these points are assuming you have a traditional hard drive, not an SSD...