When you added more RAM did you dust out the PC? There's some advice about that here
and a YouTube video here; http://uk.youtube.com/watch?v=3IvSz5NSOMk
I haven't seen one quite that dusty.
The next step is to remove unwanted Startup files;
Goto Start > Run, type in msconfig and click OK. Goto the Startup tab and review the software that attempts to load at Startup. Google will help, and any you find you don't need you can de-select.
With IE7, you can test whether any add-ons are causing it to slow down by starting IE7 with add-ons temporarily disabled. Find your IE7 icon on the Desktop, right click it, and select "Start without Add-ons". You will see an information page, then you can start surfing. Many web sites use add-ons now, so you may find some pages do not load properly, but if the otherwise load faster and new tabs work quicker, then you have narrowed the problem down to one or more add-ons.
XP runs with a number of Services, but not all of these are needed, and some can be disabled, or set to manual. The Elder Geek has a good site that lists all the Services, what they do, and how necessary they are;
He also explains how to access the Services console.
I don't usually like optimizer utilities, but one I use regularly is CCleaner from http://www.ccleaner.com It allows me to keep a check on temp files, temporary internet files, and other less easily accessible logs. The utility is free, but just note that during the install process you will be asked if you want to install the Yahoo Toolbar. I didn't.
While we all should use some security software, be sure that you only have the following running at any one time;
1] One Firewall
2] One Anti-Virus
3] One Anti-Spyware, with a couple more installed for manual checks every so often.
Having more than one of each running continuously in the background can cause conflicts, and this can slow down a computer's performance.
Disk Checking and Disk Defrag.
Personally I don't think it is necessary to run either Error Checking or Disk Defragmenter that often. Checking for errors is OK, but I don't see frequent checking will help improve the life of the disk. Once every 6 months or so for me.
XP doesn't have a problem with fragmented files that may have caused problems with earlier versions of Windows. I haven't defragmented my XP system since last year. I've just checked, and my total file fragmentation is only 3%.
See if any of that helps.
I have a Dell Dimension 8400, Pentium, 3.00GHz, 3 gigs memory. I've had it for several years. Has WIN XP with SP3.
Over time, it's getting slower and slower. I've run disk defrag frequently, Iolo System Mechanic weekly, scan for virus/adware frequently, and ran special utilities downloaded from Seagate, thinking I had a problem with the hard drive. Nothing works.
IE7.0 is sluggish, taking 10-15 seconds to shift from one open tab to another. Even typing this forum submission is a problem. I type and all of sudden, the output to the screen freezes for 10 seconds. MS Office is very slow.
So, help me out here. I've tried every utility I can think of, upgraded to the PC from 1 to 3 gigs memory. Seems like the PC is filled with molasses.