First off congrats, that is a heck of a deal and that 9200 should be able to handle most anything you throw at it. If you read the specs on most software they generally don't require near as much as what is available. Also here are the basic requirement for XP professional straight from the microsoft website.
"Here's What You Need to Use Windows XP Professional
• PC with 300 megahertz or higher processor clock speed recommended; 233 MHz minimum required (single or dual processor system);* Intel Pentium/Celeron family, or AMD K6/Athlon/Duron family, or compatible processor recommended
• 128 megabytes (MB) of RAM or higher recommended (64 MB minimum supported; may limit performance and some features)
• 1.5 gigabytes (GB) of available hard disk space*
• Super VGA (800 x 600) or higher-resolution video adapter and monitor
• CD-ROM or DVD drive
• Keyboard and Microsoft Mouse or compatible pointing device"
http://www.microsoft.com/windowsxp/pro/evaluation/sysreqs.mspxYou are getting alot of good advice you just need to piece it all together. It sounds to me that you are very competent. I am not a fan of partitions I have found them to cause problems before, but, shouldn't cause Avast! to load up. Unless it is having trouble shifting through the partitions.
XP tends to suck up about 100mb of memory if you let it. Download Rejzors rogue process terminator, you can search for it on this forum or click the link in his signature. Run his program it should eliminate all but what is essential. After doing this run some of the programs that you think Avast is slowing down. Did eliminating the rogue processes fix the problem?
In your icon tray by your clock (not by the start button). Try right clicking the icons there searching for preferences and disabling their autostart when windows starts option. Most programs don't need to run unless you intentionally run them. Having them in this taskbar eats up valuable resources. You should not have so many processes.
Set your Avast settings to normal. It is still safe.
Run your defrags on each partition, scandisk each partition and Disk clean up.
If you can reformat; and ditch the partitions, if you can't purchase a fix it program such as "System Mechanic" normally $29.00 or even the cheaper "Defender utilities" sell's at Walmart for 10.00. Google search System mechanic you may be able to download a trial version, fix your problem and then delete it. Also a program called "Partition Magic" might be able to erase the partition dividers without reformating the hard drive.
I hope we can solve your issue. I have trouble believing Avast could be the problem. It is not very resource intensive. Did this computer have another Anti-virus installed on it before you got it? Possibly Norton? If so you very well could have a conflict between the leftovers from the previous Anti-virus. They are very hard to remove all the traces of old antivirus programs. "System Mechanic" might fix this by repairing your registry. Always allow it to make a back up of your registry incase something goes wrong. Read through the posts and try all the suggestions. One might help.