A Celeron 500 really ought to be fast enough. If you only have one hard drive, that could cause problems - especially if it's fragmented, or you have a lot on it (because the information you're already storing on it will be on the fastest part of the media - the beginning, the outside of the circle), and you'll be recording on the slowest part.
I know this doesn't answer your problem directly, but audio crackling is such a random problem, it's hard to give a direct solution. Anything can cause a little glitch, when you're talking computers.
Two things that might help: When you record, cut your desktop color depth down to 256 colors. I'm not sure what you're recording with, but when I first started I had an ancient Trident PCI SVGA card, and I had my desktop running in 16 bit color. I chopped it down to 256 colors when I recorded, and my performance REALLY went up.
Another thing that could help, is to get a second hard drive for audio only. Even just a 20GB 7200RPM Western Digital drive for $80 or so would really give you a performance boost.
Or you can try overclocking your CPU. This would be accomplished by going into the BIOS setup (usually by hitting Delete when the BIOS posts - but it sounds like you might have an OEM computer, and I've always built my own, so it varies). You should see a setting for "Front Side Bus Frequency", or "Clock Speed", or something like that. It will say either "66/33" or "Default" right now. Almost every Celeron can be bumped up to a 75 MHz front side bus with just the retail Intel cooler, with no problems whatsoever. That would equal a new CPU speed of 567 MHz, and a faster system bus to boot. This would increase the overall performance of your computer.
Just some random thoughts. Performance issues may not be what's causing the audio glitches, though.
