I would run a couple good virus scans (
www.trendmicro is good free online one). I suspect what you are seeing was a trojan already on your puter since the mIRC download from approved sites is clean. Many trojans drop what is called v5.71 as part of their payload. Deleting it wont help, you need to get rid of the trojan.
I use win98se and didnt have any probs d/l or installing, and i know ppl using win98 who didnt either
As a side note to all, be sure you arent running another version of mIRC when you install, and sometimes ppl find they need to shut down everything else running (including in the background) when they install