I don't know exactly what your problem is; it could be on your ISP's side, as noted. But I did want to expand a bit on the anti-spyware program choices. There are four I know of that are seem to be in fairly common use:
* Lavasoft's Ad-Aware -- http://www.lavasoft.de/
(looks like it's the same site as in frblckstr1's link above)
* Spybot Search & Destroy -- http://www.safer-networking.org/
(choose a language here, or for English use http://www.safer-networking.org/en/index.html
* Spywareblaster -- http://www.javacoolsoftware.com/spywareblaster.html
* Microsoft's Windows Defender Beta (was Anti-spyware Beta) -- http://www.microsoft.com/athome/security/spyware/software/default.mspx
The first two tend to complement each other, so running them both is a good idea. Sometimes one detects things the other won't. Spywareblaster is a TSR-type program; run it on boot, let it load, then close it, and it will monitor your system. Microsoft's one can be set to do period scans automatically; I don't have this on my home computer because I still have WinME, but I've heard that it's also useful. So my suggestion on the spyware issue is to get these four programs and run them regularly, updating the definitions regularly as well. I do this and it's been a long time since I've seen any significant spyware on my system.
Along with the spyware scan, running some sort of virus scan regularly is also a good idea for general computer health. I'll let you look into that yourself though, if you don't already have something. A few makers I know of are Symantec/Norton, McAfee, F-Prot, and AVG. There are some online scanners, too; Panda-something is one of them, I believe.
If you can get rid of spyware and viruses, that decreases the chance of the problem being something on your computer. It could still be something else on your computer, but I'm not that big on networking, so I'm not sure I could help. I hope the additional anti-spyware links are helpful though,