hi,

you should be able to delete the file by scheduling a boot-time-scan with avast, or booting your PC to safeMode (press F8 when booting) and then doing a scan with avast/deleting it manually

But it’d be a lot better, if you’d follow a proper procedure like this:
http://www.trendmicro.com/vinfo/virusencyclo/default5.asp?VName=WORM_NACHI.B

be sure apply all Windows-Updates !!! in addition to cleaning the PC

:wink: