Online Scanning has it's pros and cons...
Thier Virus database is always uptodate, hence better virus detection dates...but, the bad thing is that connection should be fast.....
for a system like ur's, it's better u install AVG or AntiVir as they r very light.....
as far as online scanner goes, Trend Micro's House Call and CA's eTrust WebScan r the better ones....
*housecall.trendmicro.com/housecall/start_corp.asp
*www3.ca.com/securityadvisor/virusinfo/scan.aspx
Thats the 1 i have registrd as the loging here on this site.. Its a 256kpbs connection from BSNL. Dont trust them I'm not getting that much of speed.... Any way its always online so thats an added benefit because u dont have to count eggs and dimes on the dial up connec.......