HERE IS YOUR ANSWERS
u must be having an onboard gpu,which shares ram from the system memory.so consider the gpu share 32mb of sys ram,then ur effective ram will be 128-32 =96mb.
u must upgrade immediately as 128mb is the minimum requirement to run most games,even winxp.else u can go for an agp, gforce fx5200 w/128mb videoram is available for 2500.i suggest u must have 256mb minimum ram,as u have a decent processor.else u will have bottlenecs.
so it's vice to invest in a decent ramstic, which will cost u rs 1000 (for another128mb ddr)
now, dont upgrade to longhorn , u better go or windows XP. it is much better and stable than win98.and it looks and works better also.
the probs with the games may be of ::
1. overheating / check processor heat/mobocipset heat immediately after the crash. This can be done in bios system health status.
2.if everything is ok, thn it's ur onboard gpu..via/s3 integrated solutions cant play new games at all...so invest in an agp if u play games.
if it's an nforce chipset, that should play most games, still, not with much bells and whistles.
CONCLUTION :
ur system is perfect, but if u r a gamer or multimedia enthusiast, thn put at least 256 mb ram, upgrade to winxp, install a decent agp.
U will get atleast 50 to 65% performance boost if u r running win xp in this.
let me have ur thoughts pls.