Almost all PC's have no fault tolerance. If possible, get ECC memory (error correcting).
When you boot a PC, it usually checks 1 word out of every 64 MB as part of the POST (Power On Self Test). There is usually a setting in the CMOS (BIOS) to have the computer check all memory at POST, but this can take a couple of minutes with 2GB of memory.
Old memory used to have an extra bit per byte to report when there was a memory error (random memory errors occur occasionally in the best of memory chips). But to reduce the price of memory and the telephone calls from users, they removed the "Parity" check. ECC memory uses 11 bits per byte and is able to fix any 1 bit error or detect any 2 bit error. Of course, you pay more per memory stick, but if you are not returning the old computer and it is under warranty, make them replace your memory with ECC.
The sales person at the store probably did not know anything about this.
You usually see the bad RAM problems with unpacking archives, because the archives use a hash function (usually MD5) to check the results. Most work on your computer does not.
Even most memory checks do things wrong. Each section of memory should be written to, and then some parts of memory exercised for a while. The memory should be checked for changes after at least 5 minutes. Then, that section of memory should be exercised and changed to a different value and checked again 5 minutes later. Usually it takes at least 4 passes (hex A55A, count) (count, hex A55A), (hex 5AA5, count), (count, hex 5AA5). count is the location within a 64KB memory block the first two times through and the number of the memory block the last two times through.
Of course, back in the stone age when I first started working on computers, they used to frequently crash due to memory errors. Memory used to cost around $0.02 per bit (1 GB would have cost over 170 million dollars).
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