Thanks for the replies everyone.
I carried out a full scan (EZ Armour anti-virus) and lavasoft ad-aware. System is clean.
Now I've had a chance to look properly I've found out that the emails weren't coming directly to my ntl email address.
I have a domain name with email forwarding and all these emails were addressed to
support@MyDomainName.com. These were the ones being forwarded to ntl.
There were thousands of them (probably my fault that ntl's email went down!) but I have redirected them to an old Excite.com I hardly ever use. The good news is I haven't had any today so hopefully the problem has gone away.