My computer is starting to do odd things

Question

My poor old computer is starting to act up. It's a variety of things. I'll list a few, but first my question. Are there some general things I can do to give my computer a good spring cleaning. I fill it up often and archive to external drive. I am a designer and have thousands of files from many years all over the place, but organized. It feels like I just need to clean out the cobwebs and simplify to get back in control.
First of all, stuff just seems slow. Searches are slow, searches are slow, files load slow, like big photoshop files. Sometimes I think it is just sitting there doing nothing (beachballsville as we call it in the office) and suddenly it snaps to and performs the requested task. Even if I clear up 15 gigs on the drive, it doesn't change things. So I don't think it's a pure space issue.
Finder searches will work after a restart, but by the second search, it brings back no results and just keep searching even when it should be hitting a million documents. So luckily I know when everything is organized. Especially when I try to do something like a "created on" date specific search. No dice on results.
Occasionally, browsing my disk in columns mode will freak out on me and the smallest of files will take two minutes to bring up the preview thumbnail. A restart puts things to right. But it's a pain when looking through files of images for a specific image. It's fairly rare for it to have this problem, but it consistently pops up every week or two.
Once in a while, Photoshop tools windows will one place on the screen, but when I click them, I see a ghosted box the same size as the toolbox elsewhere on the screen as though my cursor thinks its elsewhere on the screen. Again, a restart fixes it. It just seems like it had a brain ****.
Programs like Photoshop and In design can take many minute to quit unless I force quit them. They will say they're not responding, but eventually they will. So if I wait for everything to close properly, it can take 10 minutes to restart my computer for all the time it takes to close the programs. That doesn't seem right.
And when I'm doing a finder search, it starts searching before I finish typing, which would be find if it let me finish typing, but it does not. It will freeze up while searching and not allow me to finish the word until it comes back with results, so don't misspell a word and type fast, or I'll get lost in beachballsville while it's spining away looking for nothing of relevance. Can I make it wait till I'm done writing the search term and click "search". It would save so much agony.
Today my active screen corners stopped responding. Guess what fixed it.
Yeah, I probably need a new machine, but it really does work pretty well most of the time. It's just that when I come across these issue at really inopportune times, it's is no frustrating.
I'm hoping someone has advice, or has corrected some of these issues in the past. I looked around the board and didn't see something that made me say, YES! that's what's happening to me too!. But I've been wrong before. Thanks for reading my tome.

Answer 1

Here's some reading for you:
Mac Tune-up: 34 Software Speedups
http://www.macworld.com/article/49489/2006/02/softwarespeed.html
52 Ways to Speed Up OS X
http://www.imafish.co.uk/articles/post/articles/130/52-ways-to-speed-up-os-x/
Tuning Mac OS X Performance
http://www.thexlab.com/faqs/performance.html
11 Ways to Optimize Your Mac's Performance
http://lowendmac.com/eubanks/07/0312.html
The Top 7 Free Utilities To Maintain A Mac.
http://mac360.com/index.php/mac360/comments/thetop_7_free_utilities_to_maintain_amac/
Mac OS X: System maintenance
http://discussions.apple.com/thread.jspa?messageID=607640