lost files

sometimes when a file is finished downloading and virus scan is compleated with no virus found the file is gone and can't be found. also the filesize of partial downloads are set to 0 when I restart Xolox. I use xp pro with Intel 500mhz 256MB ram NAV 2003 and NIS 2003. I have cable modem but, never seem to get more than 16k download speed some times as high as 40k but rare mostly less than 10k. this program keeps geeting worse.