PC stops responding

We have a mixture of NT4.0 W2K workstations, some with 128Mb and some with 256Mb. On a few of these PCs when you use the Drop down menu to open a file, open Windows explorer or My Computer the PC stops responding for between 15 – 20 seconds depending on the RAM and pagefile\swapfile in use. On my W2K workstation I have the same problem, but when I explore using my Network places, I do not get this staggered response. Anyone got any ideas. I have checked registry settings, changed Graphics acceleration, pagefile size, registry size and various different registry settings, all to no avail. HELP