Over the years since Windows XP and latterly Vista have become the Computer of use I have seen People Posting to forums about the Computer's Processor running at 100% thinking if loading the Computer with another Program running in the Background, it will Crash with a Blue screen of Death

OK, to go back to the Old Windows 99/98 setup with IE 5/5.5 The tricky Memory Card Setup which acted as a Buffer zone Plus the Virtual Memory (and Various caches) they did not add up to much, Thus a Memory Hog program such as ICQ could eat up better than 70% of the resources ,and the computer may just run on the 5 to 10% left before BSOD'ing at about 22 to 25%
ON a very good day with a "Clean computer" The resources could be high 80's to low 90's
But,as the Veterans of these older setups will tell you It took a lot of patience to Keep the Kettle boiling even at 70%
And the Forums were swamped with the Crashed browser(resource related) queries

Coming back to the Present
You'll never see that question on the Forum ...Why?

The answer is in the Technology The Memory cards are a Minimum of 256 K plus Faster Processors of 1 GHZ and Bigger Faster Hard drives allowing a bigger Virtual Memory to be set and there are more memory slots and the Graphics /Sound have been given their own dedicated circuitry/memory (the older versions relied on the RAM/ hard drive space and If memory serves correctly the ROM driver for instructions !)

Fact, a Windows98 powered computer running on a modern (32/64 bit)motherboard with a modest RAM say 512 MB a small hard drive of 40 GB can never crash on resources as the Buffers are Generous. I estimate you'd be hard pushed to get it to drop below 60% with Normal use

And Editing of the Startup List is a High Priority Item to keep the Stability (Applies to all Operating systems)

Moving on ,with XP and Vista OS Resources the issue was almost eliminated with the Task Manager Program which Prioritises Many programs together allowing a lot more internal Multitasking The 16 bit Operating system program Would virtually stop if calling on too much (The Windows95/8 Defrag program is a classic example) and take forever!
The XP/Vista defrag can be run while surfing with No Stops (So long as you keep away from Interactive sites )and takes No time (40 mins Max on my computer)

Now here is how to tell if on XP if resources are being eaten into (on Task Manager) and it'll take you a few seconds with practice
1 ) Preparation
Open Task Manage, on the top of the Window click View and on the Menu click Show Kernel Times
2) Click the Performance Tab and you'll see a "wavy Red trace in the top window running along the bottom
3) In the same window there is a Green line which is almost steady which if you look to the smaller "CPU Usage" window to the Left will light up Green bars...Yes? Now Note the red Tracer this will occasionally rise turning a bar or 2 red on the other window Pull the window about violently to make a red bar or 2
4) But the Good thing is even if it lit the box up with bright red, there is a failsafe and that is in the lower Page file Usage Window
4a) Note the yellow Line this indicates how many programs /processes are running in Memory On Mine it varies between 53 to 55 and about 550 subprocesses going on and in the Left PF Usage window I have 1 green bar lit up with 784 MB My total Memory is 2 GB Fast RAM and the same size allocated on My 160 GB Hard drive as Virtual Memory so if you add those together you'll just about light all the bars LOL
4b) The Newer processors and Motherboards also move the Data faster So a Faster FSB (front side bus)speed Helps The early boards were good for 166 to 333 Mhz The Norm now is 1000 mhz unless you have an AMD Chip Board which are rated at 2000mhz and the New Multi core chips are rated at 3000mhz.

So the Moral of the Tale is if the Performance Tabs is 100% and Red then worry (a little bit) Possible Hardware issue? Next, look below for a clue... Post the name on the Forum if you are unsure of where it relates to.
If its 100% and green then Its trying to do something .Click on the Processes tab then click on the CPU column to see what is showing at the top of the list. On Mine its always my BOINC screensaver @ 98 to 99%(set to run all the time). If its doing nothing it'll be "System Idle Process"(and there will only 1 green bar lit up) and taskmgr.exe bobbing in and out at 3%