PC’s locking

i have a number of refurbished HP’s and at least one refurb’d Dell that continually lockup.

Using MSCONFIG.EXE, I have cleared most of the programs from the startup tab (many of these computers had 65% and less available resources prior). But still these computers lock up.

One especially (the Dell) locks up after it is left alone for about 20 minutes. I cannot see that it has any power saving settings enabled. All of these computers have 96MB of RAM and are 400mhz and above.

What can be causing these problems?