It would help if you updated your system specs, but basically increasing virtual memory is only going to postpone the inevitable. If Norton is removing the trojan and it keeps coming back, either Norton is not actually completely removing the infection (likely) or you have an infected file or program you keep putting back on the machine (less likely, usually people are careful about the USB disks and CD/DVDs they put in their PC after an infection). I might suggest scanning your machine offline using a bootable virus scanner, and replacing Norton with a better (and free) A/V program like MS Security Essentials or Avast.
Bitdefender has a good (and free) bootable CD (ISO image download here
) that you can use to clean your machine offline - instructions here
(also includes USB key instructions if you wish to use that rather than a CD/DVD).