I just added 512 Mb of pc2100 RAM to total 1 Gb.

I started to have regular programs error.

I.E. and adobe acrobat are so far the one I have noticed.
Those hapen randomly.

I have removed 512Mb and those error seem to be gone.

The memory stick are compatible, I checked.

I don't know what to do?

Rgds.

Could you please state the error or blue screen you get?
And what OS do you have? Windows Xp? 98?

Most likely a memory stick is bad. Download and run memtest 86 on each stick individually.
Strider

This solution may be irrelevent because I would rather have a software memory problem than a hardware one. First, does your board use dual channel memory? Most module fabrication changes a little over time, this is a problem I have experienced. Second, if the modules are fabricated differently, they also may be rated at different voltages. To see if this is the problem, take out one module and boot the system. Play with it for a while, games, music, internet and if your system doesnt crash, that module is good. Then do the same thing with the other module by itself, if no errors, that one is good. If they both work ok, I would guess the 2 modules are incompatible with one another. Good signs of possible incompatability are the sizes of the modules. Are they the same height? Or are the positions of the RAM chips different on the modules? Make a visual inspection to see if the modules were fabricated the same. This would be my diagnosis of your problem is that the memory is incompatible, not speed wise, but because it is fabricated differently.

Is the new stick the same brand as the old one? If not then it might explain the errors. Mixing brands of RAM can cause many different types of problems. though both brands may be compatible, and even on a list of approved brands from the mobo manufacturer, mixing them can still cause compatibility problems.