You need to close the device before R finalises the file (and before it even draws on the device), e.g.
postscript("asd.ps", horizontal=FALSE)
op <- par(mfrow=c(2,2))
boxplot(cats[,2:3])
plot(Bwt, Hwt)
par(op)
dev.off() ## close the device
Once you've dealt with that, make sure .ps
files are associated with the Rampant Logic viewer rather than with Wordpad. IIRC, .ps
default to being handled by Wordpad (at least they did the last time I used Windows...)